The US secretary of state has told Russia to cease all “reckless and aggressive actions” towards Ukraine.
Antony Blinken made the comments during a trip to Kyiv on Thursday where he met top Ukrainian officials.
Speaking alongside President Volodymyr Zelensky, he said the US was looking to strengthen security co-operation and assistance between the two countries.
The visit comes just weeks after Russia deployed thousands of soldiers close to the Ukrainian border.
“We are aware that Russia has withdrawn some forces from the border… but we also see that significant forces remain there [and] significant equipment remains there,” Mr Blinken said.
The top US diplomat said Washington was “monitoring the situation very, very closely”.
He did not provide further details of any additional security support the US was considering.
Barbara Plett Usher, the BBC’s State Department correspondent, says the visit was intended to signal that Ukraine is a high foreign policy priority for President Joe Biden’s administration.
‘Irresponsible and destabilising’
Mr Blinken travelled to Ukraine following a meeting of foreign ministers of the G7 group of industrialised nations in London earlier this week.
A statement issued after the two-day talks criticised Moscow for its “irresponsible and destabilising behaviour”, particularly against Ukraine, and for cyber-attacks.
Mr Blinken echoed those sentiments on Thursday while appearing alongside President Zelensky, who said his country was under threat from Russian troops massed near its border.
The US and Russia have been long-term adversaries and remain far apart on major issues.
In February, US President Joe Biden said he had made it clear to Russian President Vladimir Putin “that the days of the United States rolling over in the face of Russia’s aggressive actions [were] over”.
But Mr Biden’s predecessor, Republican Donald Trump, said during his presidential campaign that he would get along with Mr Putin. He sought to improve ties while in office, a stance at odds with traditional Republican party policy.