KIEV: 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.”

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.

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”.

“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.