MOSCOW • Tensions in the long-running conflict between Russia and Ukraine have spiked in recent days. Here is what we know about the escalation:
Kiev has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions since 2014, following Moscow's annexation of Crimea.
After several years of serious clashes - claiming some 13,000 lives - the fighting subsided and a ceasefire agreed last year brought relative calm. But fighting has sharply escalated since the start of the year, mainly involving artillery and mortar fire.
At least 29 Ukrainian soldiers have been killed since the beginning of January, compared with 50 in all of last year.
The separatists have said that more than 20 of their fighters have been killed this year.
RUSSIAN TROOP BUILD-UP
As fighting grew, Kiev accused Moscow of a massive military build-up on its borders, with an official saying on Monday that Russia had 41,000 troops on Ukraine's eastern frontier and 42,000 soldiers in Crimea.
Russia did not deny the troop movements, but insisted it was "not threatening anyone".
On Tuesday, Russian Defence Minister Sergei Shoigu confirmed Moscow had sent thousands of troops to its western and southern borders, saying this was in response to threatening North Atlantic Treaty Organisation (Nato) actions.
Mr Shoigu said two armies and three airborne units were deployed in the last three weeks to conduct training exercises that would last another two weeks. Two Russian military analysts - Mr Vasily Kashin at the Higher School of Economics and independent expert Alexander Golts - said this would represent some 100,000 troops.
WEST RALLIES BEHIND KIEV
Kiev's Western allies have rallied behind Ukraine, whose pro-West leanings and 2008 application to join Nato have infuriated former Soviet master Moscow.
The US and European powers warned Moscow against further action, while US forces in Europe raised their alert status.
In the latest show of support, Nato secretary-general Jens Stoltenberg on Tuesday told Russia to end its "unjustified" military build-up as Kiev and Washington's top diplomats held talks in Brussels.
Ukraine is pressing Western powers for "practical" support, with President Volodymyr Zelensky urging Nato to speed up the country's membership of the alliance.
Observers have pointed to several potential reasons for the escalation. Many say both Moscow and Kiev may be testing new US President Joe Biden to see how far he is willing to go to defend Washington's ally and confront Russia.
Others suggest Russia could be sending a signal to Ukraine to back off after Kiev imposed sanctions on lawmaker and businessman Viktor Medvedchuk, a close ally of Mr Putin, and banned three pro-Russian television channels linked to Mr Medvedchuk. And some say the Kremlin could be looking to boost patriotic feelings ahead of parliamentary elections in September.
Analysts say a full-scale invasion of Ukraine by Russia or a Nato-backed assault by Kiev on the separatists seems unlikely for now, but that tensions have reached a surprising level. Peace talks may be an option, but efforts mediated by France and Germany - the so-called Normandy format - have made little progress in recent years.
New negotiations do not seem to be on offer. Mr Zelensky's office said he had sent a query late last month requesting talks with Mr Putin, but had yet to receive an answer.