US President Joe Biden affirmed his "unwavering support" for Ukraine in a call to President Volodymyr Zelensky on Friday after Kiev accused Moscow of building up military forces on its border.

The call, which was Biden's first conversation with Zelensky since the US leader's inauguration in January, came after Russia warned the West earlier Friday against sending troops to Ukraine to buttress its ally.

It also came as tensions between the United States and Russia have hit rock bottom after Biden last month infuriated Moscow by agreeing with a description of his Russian counterpart Vladimir Putin as a "killer".

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

Zelensky, who this week accused Russia of massing troops on Ukraine's border, said in a video released by his office that "President Biden assured me that Ukraine will never be left alone against Russian aggression."

Ukraine has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions since 2014
Ukraine has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions since 2014 AFP / Anatolii STEPANOV

Weeks of renewed frontline clashes have shredded a ceasefire and raised fears of an escalation of the simmering conflict in eastern Ukraine, where government forces are battling Russia-backed separatists.

Speaking to reporters on Friday, 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," 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," Peskov said.

Moscow and Kiev this week blamed each other for a rise in violence along the frontline
Moscow and Kiev this week blamed each other for a rise in violence along the frontline AFP / Anatolii STEPANOV

On Friday, Russia said its armed forces would hold military exercises close to Ukraine's border in the country's south to practise defence against attack drones.

More than 50 battalion combat teams comprising 15,000 people will take part in those exercises and practise "interaction with electronic warfare and air defence units", the defence military told reporters.

British Foreign Secretary Dominic Raab said on Twitter Friday that he had called his Ukrainian counterpart Dmytro Kuleba to reaffirm "UK support for Ukraine's sovereignty & territorial integrity".

Moscow and Kiev this week blamed each other for a rise in violence along the frontline
Moscow and Kiev this week blamed each other for a rise in violence along the frontline AFP / Anatolii STEPANOV

"We are gravely concerned about Russian military activity which threatens Ukraine," he wrote.

But Steven Pifer, a former US ambassador to Ukraine and a fellow at the Robert Bosch Academy in Berlin, said movements by Russia's forces were "90 percent to rattle the Ukrainians".

"I don't see the value to the Russians of taking more territory," he told AFP.

Kiev 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.

Zelensky said on Thursday 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.

Ruslan Khomchak, chief of the general staff of the Ukrainian Armed Forces, this week 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 said Russia is at liberty to move troops on its own territory.

"Russia is not a participant of the conflict," Peskov said on Friday, 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.

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.