Tensions between Ukraine and Russia are at their highest in years, with reports of a Russian troop build-up near Russia's borders with Ukraine fuelling fears over Moscow's intentions.
Ukraine warns Russia is trying to destabilise the country ahead of any planned military invasion. Western powers have repeatedly warned Russia in recent weeks against further aggressive moves against Ukraine.
The Kremlin denies it is planning to attack and argues that NATO support for Ukraine —including increased weapons supplies and military training — constitutes a growing threat on Russia's western flank.
The picture is complicated — but here's a breakdown of what we know.
What's the current situation on the border?
The United States and NATO have in recent weeks described the movements and concentrations of troops in and around Ukraine as "unusual."
Last month, satellite photos revealed Russian hardware — including self-propelled guns, battle tanks and infantry fighting vehicles — on the move at a training ground roughly 300 km from the border. But little other information has been made public to back up the allegation by Western powers of an increased threat.
Many of Russia's military bases are to the west of the vast country — the direction from which history suggests any threats are most likely to come. Russia's Defense Ministry said on December 1 that it had started "regular" winter military drills in its southern region, parts of which border Ukraine. The exercises involve more than 10,000 troops, the ministry said.
Meanwhile, Ukraine's eastern Donetsk and Luhansk regions bordering Russia, an area known as Donbas, have been under the control of Russian-backed separatists since 2014. Russian forces are also present in the area, referred to by Ukraine as "temporarily occupied territories," although Russia denies it.
The front lines of the conflict have barely moved in five years, but there are frequent small-scale clashes and sniper attacks. Russia was angered when Ukrainian forces deployed a Turkish-made combat drone for the first time in October to strike a position held by the pro-Russian separatists.
Russia also has forces numbering in the tens of thousands at its massive naval base in Crimea, the Ukrainian territory it annexed in 2014. The Crimean peninsula, which lies to the south of the rest of Ukraine, is now connected by a road bridge to mainland Russia.
What's the history of the conflict between Ukraine and Russia?
Tensions between Ukraine and Russia, both former Soviet states, escalated in late 2013 over a landmark political and trade deal with the European Union. After the pro-Russian then-President, Viktor Yanukovych, suspended the talks — reportedly under pressure from Moscow — weeks of protests in Kiev erupted into violence.
Then, in March 2014, Russia annexed Crimea, an autonomous peninsula in southern Ukraine with strong Russian loyalties, on the pretext that it was defending its interests and those of Russian-speaking citizens. First, thousands of Russian-speaking troops, dubbed "little green men" and later acknowledged by Moscow to be Russian soldiers, poured into the Crimean peninsula. Within days, Russia completed its annexation in a referendum that was slammed by Ukraine and most of the world as illegitimate.
Shortly afterwards, pro-Russian separatists in Ukraine's Donetsk and Luhansk regions declared their independence from Kiev, prompting months of heavy fighting. Despite Kiev and Moscow signing a peace deal in Minsk in 2015, brokered by France and Germany, there have been repeated ceasefire violations.
According to the latest UN figures, there have been more than 3,000 conflict-related civilian deaths in eastern Ukraine since March 2014.
The European Union and US have imposed a series of measures in response to Russia's actions in Crimea and eastern Ukraine, including economic sanctions targeting individuals, entities and specific sectors of the Russian economy.
The Kremlin accuses Ukraine of stirring up tensions in the country's east and of violating the Minsk ceasefire agreement.
What's Russia's view?
The Kremlin has repeatedly denied that Russia plans on invading Ukraine, insisting Russia does not pose a threat to anyone and that the country moving troops across its own territory should not be cause for alarm.
Moscow sees the growing support for Ukraine from NATO — in terms of weaponry, training and personnel — as a threat to its own security. It has also accused Ukraine of boosting its own troop numbers in preparation for an attempt to retake the Donbas region, an allegation Ukraine has denied.
Russian President Vladimir Putin has called for specific legal agreements that would rule out any further NATO expansion eastwards towards Russia's borders, saying the West has not lived up to its previous verbal assurances.
Putin has also said that NATO deploying sophisticated weapons in Ukraine, such as missile systems, would be crossing a "red line" for Russia, amid concern in Moscow that Ukraine is being increasingly armed by NATO powers.
Kremlin spokesman Dmitry Peskov said last month that weapons and military advisers were already being supplied to Ukraine by the US and other NATO member states. "And all this, of course, leads to a further aggravation of the situation on the border line," he said.
If the US and its NATO allies do not change course in Ukraine, Russian Foreign Minister Sergey Lavrov has warned that Moscow has the "right to choose ways to ensure its legitimate security interests."
What is Ukraine's view?
Ukraine's government insists that Moscow cannot prevent Kiev from building closer ties with NATO if it chooses.
"Russia cannot stop Ukraine from getting closer with NATO and has no right to have any say in relevant discussions," the Foreign Ministry said in a statement to CNN, in response to Russian calls for NATO to halt its eastward expansion.
"Any Russian proposals to discuss with NATO or the US any so-called guarantees that the Alliance would not expand to the East are illegitimate," it added.
Ukraine insists Russia is seeking to destabilise the country with the country's president, Volodymyr Zelensky, recently saying a coup plot, involving Ukrainians and Russians, has been uncovered.
Ukrainian Foreign Minister, Dmytro Kuleba, warned that a planned coup could be part of Russia's plan ahead of a military invasion. "External military pressure goes hand in hand with domestic destabilisation of the country," he said.
Tensions between the two countries have been exacerbated by a deepening Ukrainian energy crisis that Kiev believes Moscow has purposefully provoked.
At the same time, Zelensky's government faces challenges on many fronts. The government's popularity has stagnated amid multiple domestic political challenges, including a third wave of COVID-19 infections in recent weeks and a struggling economy.
Many people are also unhappy that the government hasn't yet delivered on benefits it promised and ended the conflict in the country's east. Anti-government protests have taken place in Kiev.
What does NATO say?
Secretary General Jens Stoltenberg has said "there will be a high price to pay for Russia" if it once again invades Ukraine, a NATO partner.
"We have a wide range of options: economic sanctions, financial sanctions, political restrictions," said Stoltenberg, in a December 1 interview with CNN.
After Russia invaded Ukraine in 2014, NATO increased its defences "with combat-ready battle-groups in the eastern part of the alliance, in the Baltic countries, in Latvia … but also in the Black Sea region," Stoltenberg said.
Ukraine is not a NATO member, and therefore doesn't have the same security guarantees as NATO members.
But Stoltenberg left the possibility of Ukraine becoming a NATO member on the table, saying that Russia does not have the right to tell Ukraine that it cannot pursue NATO membership.
What does the United States say?
The US and its NATO allies have "deep concerns" regarding the "aggressive posture" taken recently by Russia towards Ukraine, a strategic ally of the US, Secretary of State Antony Blinken said at an OSCE summit in Sweden.
He has previously said Russia is massing combat troops along Ukraine's border and is putting in place the capacity to invade at short notice, if it chooses
Blinken warned Russia that "any renewed aggression can trigger serious consequences."
The Biden administration is also weighing sending military advisers and new equipment including weaponry to Ukraine to prepare allies for a possible Russian invasion, multiple sources familiar with the deliberations told CNN in November.
The Obama administration was taken by surprise when Russia invaded Crimea in 2014 and backed an insurgency in eastern Ukraine's Donbas region. US officials say they are determined not to be caught out by another Russian military operation.
"Our concern is that Russia may make a serious mistake of attempting to rehash what it undertook back in 2014, when it amassed forces along the border, crossed into sovereign Ukrainian territory and did so claiming falsely that it was provoked," Blinken said in November.
What other factors are at play?
Another big issue revolves around energy supply. Ukraine views the controversial Nord Stream 2 pipeline — connecting Russian gas supplies directly to Germany — as a threat to its own security.
Nord Stream 2 is one of two pipelines that Russia has laid underwater in the Baltic Sea in addition to its traditional land-based pipeline network that runs through eastern Europe, including Ukraine.
Kiev views the pipelines across Ukraine as an element of protection against an invasion by Russia, since any military action could potentially disrupt the vital flow of gas to Europe.
Analysts and US lawmakers have raised concerns that Nord Stream 2 will increase European dependence on Russian gas and could allow Moscow to selectively target countries such as Ukraine with energy cut-offs, without broader disruption to European supplies. Bypassing eastern European countries also means those nations would be deprived of lucrative transit fees Russia would otherwise pay.
In May 2021, the Biden administration waived sanctions on the company behind Nord Stream 2, effectively giving it the green light. US officials say the move was in the interest of US national security as it sought to rebuild frayed relations with Germany.
The US last month imposed new sanctions on a Russian-linked entity and a vessel linked to Nord Stream 2. Some US senators have called for further sanctions to be imposed to prevent Russia using the pipeline as a weapon; Ukraine too has called for tougher measures.
"We see different ways how this project should be prevented from being weaponized by Russia. We differ when it comes to the volume and the amount of sanctions needed to protect Europe from Nord Stream 2," Kuleba, the Ukrainian Foreign Minister, told CNN.