The deployment to the former Soviet bloc country was being made "in the spirit of solidarity", said a defence ministry statement. It would support its intelligence, surveillance and reconnaissance efforts.

Although Lithuania is a NATO member, the personnel will not be sent through the Western military alliance but bilaterally, the statement added.

"The UK and Lithuania have shared concerns about Russia's military build-up on Ukraine's border and the unfolding migrant crisis on Lithuania's border with Belarus," Defence Secretary Ben Wallace said in a statement.

"I have now directed the bilateral deployment of a small number of military personnel to address these border pressures."

Britain has announced a flurry of troop deployments in recent weeks, sending an additional 350 soldiers to Poland to help shore up NATO's eastern flank against any perceived Russian aggression.

They join the 150 soldiers Britain already contributes to NATO's enhanced forward presence there.

London also sent another 150 soldiers to Poland in December in response to the migrant crisis on the Belarusian border.

It is nearly doubling its NATO deployment in Lithuania's Baltic neighbour Estonia -- from 900 to 1,750 -- and until recently had a smaller force in Ukraine for training on UK anti-tank missiles.

Prime Minister Boris Johnson said 1,000 troops would be placed on readiness to deploy at short notice for a "humanitarian response" in the region if required, his office said last week.

US intelligence has warned that Russia, which has massed more than 100,000 troops around Ukraine's borders, could invade its western neighbour within days.