US President Joe Biden has expressed firm support for Ukraine in a call to President Volodymyr Zelensky amid a buildup of Russian armed forces on the country's border, the White House said.
Mr 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.
It comes after Russia warned the West against sending troops to Ukraine to strengthen its ally, after Kiev accused Moscow of building up a military presence on its border.
Mr Zelensky said Russia was massing troops on the border and the US pledged to stand by Ukraine in the event of Russian "aggression".
Weeks of renewed frontline clashes have raised fears of an escalation of the simmering conflict in eastern Ukraine, where government forces are battling pro-Russian separatists.
Kremlin spokesman Dmitry Peskov said Russia would be forced to respond if the US sent troops.
"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," Mr Peskov told reporters.
He declined to specify which measures would be adopted, while insisting that Russia was not making moves to threaten Ukraine.
"Russia is not threatening anyone, it has never threatened anyone," Mr Peskov said.
Earlier this week, the US warned Russia against "intimidating" Ukraine, with Secretary of Defense Lloyd Austin and Secretary of State Antony Blinken calling their Ukrainian counterparts to stress support.
The Department of Defense said US forces in Europe had raised their alert status following the "recent escalations of Russian aggression in eastern Ukraine".
Ukraine has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions since 2014, following Moscow's annexation of the Crimean peninsula after an uprising that ousted Ukraine's Kremlin-friendly president Viktor Yanukovych.
Moscow and Kiev this week blamed each other for a rise in violence along the frontline that has undermined a ceasefire brokered last July.
President Zelensky said that 20 Ukrainian servicemen had been killed since the start of the year.
Ukraine's military intelligence accused Russia of preparing to "expand its military presence" in the separatist-controlled regions.
Earlier this week, Ruslan Khomchak, chief of the general staff of the Ukrainian Armed Forces, said that more than 2,000 Russian military instructors and advisers, were currently stationed in eastern Ukraine.
Russia has repeatedly denied sending troops and arms to support the separatists and the Kremlin this week said that Russia is at liberty to move troops on its own territory.
"Russia is not a participant of the conflict," Mr Peskov said today, accusing Ukraine's armed forces of "multiple" provocations in the region.
A senior Russian official dismissed reports of Russia planning an attack on Ukraine as "fake".
"Russia is not interested in any conflict with Ukraine, especially a military one," deputy foreign minister Andrei Rudenko told state news agency RIA Novosti.
Together with France, Germany and Ukraine, Russia is part of the Normandy format of countries that have sought to resolve the conflict and in 2015 agreed the Minsk accords to reduce the fighting.
Mr Zelensky was elected in 2019 on promises of ending the conflict, but critics say a shaky ceasefire has been his only tangible achievement.
The fighting has claimed more than 13,000 lives since 2014, according to the United Nations.
