Russia warned the West on Friday against sending troops to Ukraine to buttress its ally, after Kiev accused Moscow of building up a military presence on its border.

The United States had pledged on Thursday to stand by Ukraine in the event of Russian "aggression," after Ukrainian President Volodymyr Zelensky accused Russia of massing troops on the border. 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 pro-Russian 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.

On Thursday, the US warned Russia against "intimidating" Ukraine, with both Defence Secretary Lloyd Austin and Secretary of State Antony Blinken calling their Ukrainian counterparts to stress support.

The Pentagon said this week US forces in Europe had raised their alert status following the "recent escalations of Russian aggression in eastern Ukraine".

Increase in fighting

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. 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 this week said that 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.

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.

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.