Biden "affirmed the United States' unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression in the Donbas and Crimea," the White House said in a statement.

The call came after Kiev accused Russia of massing troops along its border and in Crimea, the Ukraine territory that Moscow seized in 2014.

Since that year Ukraine has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions.

Top US leaders, including Secretary of State Antony Blinken and Defense Secretary Lloyd Austin, have called their Ukraine counterparts this week to signal US backing against what the State Department called "escalations of Russian aggressive and provocative actions in eastern Ukraine."

US troops in Europe have gone into a heightened "potential imminent crisis watch" status this week due to the Russian troop movements, The New York Times reported.

Earlier Friday Russia warned the west against mobilizing troops to help Kiev.

"There is no doubt such a scenario would lead to a further increase in tensions close to Russia's borders. Of course, this would call for additional measures from the Russian side to ensure its security," Kremlin spokesman Dmitry Peskov told reporters.

In the call Biden also emphasized to Zelensky Washington's commitment to revitalize the two countries' "strategic partnership," stressing that Zelensky's reforms and actions against corruption "are central to Ukraine's Euro-Atlantic aspirations," the White House said.