Dozens of troop carriers and missile launchers sit on flatbed wagons lining up along tracks running through southern Russia, in a region bordering Ukraine.
Tanks are parked in columns beside the railway, which runs parallel to the M4 highway. Military trucks rumble past, heading toward the southern city of Rostov-on-Don, close to the border.
Ukraine and Western countries accuse Russia of sending troops and heavy weapons to support proxy fighters who seized a swathe of the eastern Donbass region in 2014.
Moscow denies it is part of the conflict in eastern Ukraine and says it provides only humanitarian and political support to the separatists.
The recent deployment of hardware close to Ukraine's border, captured this week on video seen by Reuters, is what a source close to both the Kremlin and to pro-Russian separatists said was a deliberate show of force at a time of rising tensions between the former Soviet states.
Two sources close to separatist leaders in Donbass said skirmishes were increasing around the separatist-held city of Donetsk, and Kyiv has reported that five of its soldiers have been killed in fighting there this week.
But the military buildup is not a sign of a major escalation in the conflict, according to the source and the two people close to the separatists.
"Moscow is doing it openly, deliberately uncovering tanks in the daytime. This is not how war is prepared, this is just a show of strength," said the source close to the Kremlin and to separatists.
When asked about the footage of military equipment, Kremlin spokesman Dmitry Peskov declined comment and referred Reuters to the Defence Ministry. The Defence Ministry did not reply to requests for comment.
In recent weeks, Ukraine has displayed caution.
Since the build-up, it has said it is ready to defend itself against any attack in its eastern Donbass region, a mainly Russian-speaking area that fell to pro-Russian separatists who attacked in 2014.
Kyiv says 14,000 people have been killed in the fighting.
Washington and the NATO alliance have accused Russia of a "provocative" build-up, with unverified images of army trucks and missile launchers, taken from the ground and by satellite, widely circulating on social media.
For its part, Russia accuses Ukraine of unspecified provocation and says it can move military assets around its country as it sees fit.
A senior U.S. administration official said they believed Russia's mobilisation was meant to test Zelenskiy and also perhaps challenge the resolve of the administration of President Joe Biden, who pledged "unwavering support" for his Ukrainian counterpart Volodymyr Zelenskiy last week. read more
Ukraine is no military match for Russia and it sits outside the NATO alliance. Kyiv lost its Black Sea region of Crimea to Russian troops in 2014 without a fight.
Ukraine turns to the United States and the European Union for support against Russia, but beyond sanctions it is unclear what it can expect from its Western allies. Many years of lobbying for NATO membership have yet to bear fruit.
For Russian President Vladimir Putin, the threat of further economic sanctions by the West looms if countries deem Russia responsible for stoking the conflict.
Russian state banks and oil firms have been hit hard with U.S.-led sanctions, and, while reserves remain healthy at $575 billion, they could take a significant hit if the punishments are harsh.
Ukrainian security chief Oleksiy Danilov told Reuters he believed Putin was using the military build-up to distract Russians from an internal opposition movement led by jailed Kremlin critic Alexei Navalny, ahead of September parliamentary elections.
"Today we are calculating all scenarios from the most critical to the most light, and I emphasize once again - it all depends on Putin," Danilov said on Wednesday.
Peskov denied there was any link to internal opposition, adding that Russia had to be cautious towards its "restless" neighbour Ukraine while repeating that the military build-up should not be a worry to anyone.
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