Moscow - The West is growing increasingly worried that Russia is planning a winter invasion of Ukraine.
Top US diplomats, who have joined NATO and Europe in warning against military action, meet their Russian counterparts on Thursday in Sweden.
- What sparked the concern? -
In late October, videos began circulating on social media showing Russia moving troops, tanks and missiles towards the Ukrainian border.
Ukrainian officials said at the time that Russia had moved about 115,000 soldiers to the area.
Kiev and its Western allies have long accused Moscow of sending troops and weapons across the border to support pro-Russian separatists that seized two eastern regions in 2014 shortly after Russia annexed Crimea.
The Kremlin denies those claims.
US Secretary of State Antony Blinken said last month Washington was concerned about "unusual" Russian activity and warned Moscow against making another "serious mistake" as in 2014.
- Why would Russia act now? -
Russia previously built up forces on the Ukrainian border this spring, with some analysts speculating the idea was to extract diplomatic benefits.
Russia pulled back shortly after US President Joe Biden and his Russian counterpart Vladimir Putin announced a summit.
Some experts have said Putin could be trying the same trick again, as talks over another summit continue.
Others have suggested Ukraine angered Russia by using drones made by NATO member Turkey.
Russia's build-up came just as the Ukrainian army released footage of what it said was its first use of a Turkish drone against the separatists.
- What might Russia hope to achieve? -
Putin has accused the West of ignoring Russia's "red lines" by holding drills in the Black Sea and sending Kiev modern weaponry, demanding "legal guarantees" from NATO that it would not expand eastwards.
In an article in July he labelled Ukraine a historic heartland of the Slavic people and warned the West not to try to turn it against Russia.
"We will never allow our historical territories and people close to us living there to be used against Russia," Putin wrote.
"And to those who will undertake such an attempt, I would like to say that this way they will destroy their own country."
Ukraine's Zelensky visits eastern frontline (April 2021)
- How has the West responded? -
The US, NATO and Europe have repeatedly warned Russia against taking military action, and Blinken will meet Russian Foreign Minister Sergei Lavrov in Stockholm to discuss the conflict.
Blinken has expressed concern that Russia might seek to claim it was provoked into military action.
Lavrov on Wednesday warned against a Ukrainian "military adventure" after his ministry said Kiev had deployed some 125,000 troops to the east.
What is NATO?
- What are the chances of an invasion? -
Russia has dismissed claims of a planned invasion as "hysteria" and Putin this week compared the West to a boy crying wolf, saying that Moscow did not invade this spring after similar concerns.
Alexander Baunov of the Carnegie Moscow Centre told AFP he could "hardly imagine an invasion without a reason" and questioned what Russia would have to gain.
Some of Russia's other military operations, however, developed quickly.
In 2008, Russia bombed targets across Georgia after its then-president Mikheil Saakashvili sent troops to fight separatists.
Last month, Russia's SVR foreign intelligence service drew a direct line between Georgia and Ukraine.
It said in a statement that the 2008 conflict came as Georgia made overtures to join NATO and warned Ukrainian President Volodymyr Zelensky about repeating Saakashvili's move.
"It cost him a high price," the SVR said.
Putin says West 'escalating' Ukraine conflict (Nov 18)