US Secretary of State Antony Blinken urged Russia to “stop its dangerous and aggressive actions" towards Ukraine during a meeting with Ukrainian President Volodymyr Zelenskyy in Kyiv on Thursday.
The warning comes as tensions on the border between the two nations remain high.
Blinken said that, although Russia has reduced its military presence on the border with Ukraine, it maintains "significant forces" there.
"Russia has capabilities to, in a short period of time, take aggressive action if it makes that decision," Blinken told journalists after the meeting
Zelenskyy said the Russian military threat against Ukraine still persists, as Moscow has only slightly reduced the number of its troops deployed on the country's borders.
"We consider that the reduction (of the number of Russian troops) is too slow, that's why the threat may still exist," Zelenskyy said at a joint press conference with Blinken.
Earlier Blinken met with his Ukrainian counterpart, Dmytro Kuleba, telling him that he was there to “reaffirm strongly” Washington's commitment to Ukraine's “sovereignty, territorial integrity and independence.”
He added that the U.S. was committed “to work with you and continue to strengthen your own democracy, building institutions, advancing your reforms against corruption.”
Moscow had deployed tens of thousands of soldiers on the borders of Ukraine, officially for "military exercises" that made the country and the West fear a possible offensive or even an invasion.
The massive military deployment was accompanied by a resurgence of violence in the conflict with pro-Russian separatists in eastern Ukraine and major maritime manoeuvres in the Black Sea and the Crimean peninsula.
Russia had assured that these troop movements "did not threaten anyone", and that it was a response to "aggressive" operations of NATO in Eastern Europe.
Washington, Brussels and NATO have multiplied statements of support for Kyiv in this context, but have not acceded to the Ukrainian request to accelerate its entry into the Atlantic Alliance, a red line for Moscow.
Zelenskyy thanked his American guest for the support of the United States against Russia, saying that "much had been done to stop the escalation."