Russia is sending two battalions of S-400 surface-to-air missile systems to Belarus to join military drills there next month, Interfax, an independent news agency in Moscow, said on Friday.

Russia and Belarus will rehearse repelling an external attack when they hold joint military drills February 10 to 20 in Belarus, both sides announced on Tuesday, at a time of acute tensions with the West over neighboring Ukraine.

Russia's defense ministry said two S-400 battalions - which typically include eight anti-aircraft missile systems each - have started moving to Belarus from Russia's Far East by train.

Belarusian leader Alexander Lukashenko said the drills were needed as Ukraine had built up troops near Belarus.

The American leader had vowed a "severe" response to any invasion of Ukraine.

U.S. President Joe Biden on Wednesday warned that U.S. allies and partners are ready to impose severe costs and significant harm on Russia and the Russian economy. "If they actually do what they're capable of doing with the force they've massed on the Ukrainian border, it is going to be a disaster for Russia," he told reporters.

On Tuesday, the Russian defense ministry's announcement to deploy more than 140 warships and supporting vessels this month and in February to the Atlantic, Pacific, Arctic and Mediterranean was followed by a condemnation from the Kremlin of what it described as "destabilizing" remarks from Biden.

In response, Moscow insists it has no plans to invade but laid down a series of demands, including a ban on Ukraine joining The North Atlantic Treaty Organization (NATO), in exchange for de-escalation. Washington has rejected Moscow's demand as "non-starters" and NATO chief Jens Stoltenberg this week insisted that the alliance "will not compromise on core principles such as the right for each nation to choose its own path".

U.S. Secretary of State Antony Blinken was on a diplomatic tour that took him from France to Germany this week. Then he will have a talk with Russian Foreign Minister Sergei Laurov.

(With input from agencies)