"U.S. citizens are advised of concerning reports of unusual Russian military activity near Ukraine’s borders and in occupied Crimea," the embassy said in a statement posted on its website.
It advised Americans against traveling in Crimea "due to abuses by Russian occupation authorities, in the eastern parts of the Donetsk and Luhansk oblasts due to armed conflict, and for all of Ukraine due to COVID-19."
"U.S. citizens are reminded the security conditions along the border may change with little or no notice," the statement said.
US officials have voiced growing concerns in recent weeks that a Russian military buildup near Ukraine could signal plans by Moscow to invade its ex-Soviet neighbour again, news outlets have reported.
The Kremlin has denied any such intention and has accused Ukraine and its Western backers of making the claims to cover up their own allegedly aggressive designs, the AP news agency reported.
Polish Foreign Minister Zbigniew Rau on Tuesday warned that Russia may be plotting a new attack on neighboring Ukraine as Moscow builds up forces along their shared border.
According to the Ukrainian intelligence service, Russia has amassed more than 90,000 troops at its border with Ukraine, and could be preparing for an attack at the end of January or beginning of February, public broadcaster Polish Radio’s IAR news agency has reported.
US Secretary of State Antony Blinken said earlier this month that "a rehash" of the 2014 aggression against Ukraine would be "a serious mistake" for Russia.
In 2014, Russia annexed the Crimean Peninsula from Ukraine and fomented a separatist conflict in that country's eastern Donbas region, leading to a wave of EU and US sanctions against Moscow and Russian officials.
Source: usembassy.gov, AP