The US State Department on Monday branded as an "unprovoked escalation" reported Russian plans to block parts of the Black Sea, which could ultimately impact access to Ukrainian ports.
Russian state media have reported that Moscow intends to close parts of the Black Sea to foreign military and official ships for six months.
Such a move could affect access to Ukrainian ports in the Sea of Azov, which is connected to the Black Sea through the Kerch Strait, on the eastern tip of the Crimean peninsula annexed by Russia in 2014.
"This represents yet another unprovoked escalation in Moscow's ongoing campaign to undermine and destabilize Ukraine," State Department spokesman Ned Price said in a statement.
"This development is particularly troubling amid credible reports of Russian troop buildup in occupied Crimea and around Ukraine's borders, now at levels not seen since Russia's invasion in 2014," he added.
Tensions between Russia and the West have been escalating in recent weeks following an uptick in fighting between Ukraine's army and pro-Russia separatists in eastern Ukraine.
Moscow has amassed tens of thousands of troops along Ukraine's northern and eastern borders, as well as in annexed Crimea, sparking warnings from NATO.