A worrying new satellite image has shown Russia amassing military near the border where tensions with Ukraine are mounting.
The image taken by Planet Labs Inc earlier this month shows a camp of Russian troops just 280km away from Ukrainian territories.
According to German newspaper Der Spiegel, the image also shows more than 1000 military vehicles as well as accommodation.
Ruslan Leviev, an open source analyst with Conflict Intelligence Team, told the newspaper it was the "greatest concentration of Russian armed forces since 2014 and 2015" near the Ukrainian border.
He added it was a show of force by Russia.
Risk of escalation 'evident'
The satellite image has emerged as top EU diplomat Joseph Borrell said Russia had concentrated more than 150,000 troops on Ukraine's border in annexed Crimea, with the risk of further escalation "evident".
Kiev has been locked in a conflict with Russian-backed separatists since 2014, and earlier this month Ukrainian officials claimed Russia was flexing its military muscle on the territories controlled by the separatists – rebel Ukrainians who have formed a militia under the command of pro-Russian commander, Igor Girkin.
The US Pentagon says the current military build-up was larger than in 2014, and it was not clear if it was amassing troops for training purposes.
US expresses 'deep concern' over Russia's plan
US State Department spokesman Ned Price said in a statement the US had "deep concern" over Russia's plans to also block foreign naval ships and other vessels in parts of the Black Sea.
"Russia has a history of taking aggressive actions against Ukrainian vessels and impeding access to Ukraine's ports in the Sea of Azov, impacting Ukraine's international commerce," the statement on April 19 said.
"This represents yet anther unprovoked escalation in Moscow's ongoing campaign to undermine and destabilise Ukraine.
"This development is particularly troubling amid credible reports of Russian troop build-up in occupied Crimea and around Ukraine's borders, now at levels not seen since Russia's invasion in 2014."
Mr Price reaffirmed the United States' "unwavering" support for Ukraine's sovereignty and territorial integrity within its internationally recognised borders and its territorial waters.
"The United States does not, and will never, recognise Russia's purported annexation of Crimea," the statement said.
Russia restricted the movement of foreign warships in the Black Sea as two Russian warships were seen transiting the Bosphorus en route to the Black Sea on Saturday as Moscow beefed up its naval presence.
Ukrainian Foreign Minister Dmytro Kuleba has since addressed EU foreign ministers and called on the EU to impose new sanctions on Russia.
The US Federal Aviation Administration has also urged airlines to exercise "extreme caution" when flying near the Ukraine-Russian border, citing potential flight safety risks.