Ukraine has begun calling up reservists and deploying troops to the border to counter what it says is the growing threat of a Russian invasion.

President Petro Poroshenko, who last week declared martial law in 10 regions, announced the military moves on Monday in response to a “sharp increase in Russian forces along our borders” and in the Crimean peninsula.

The day before, Mr Poroshenko claimed that Russia was trying to capture the major port of Mariupol to create a land corridor to Crimea.

Vladimir Putin's spokesman called this statement “absolutely absurd” on Monday and accused Ukraine's leader of “provoking tensions” before the presidential election there in March.

The escalation stems from when Russian forces fired upon and seized three Ukrainian naval ships attempting to pass the Kerch Strait on November 25. The vessels were heading for Ukrainian ports in the Sea of Azov, where Kiev has announced plans for a naval base.

The speaker of the Russian parliament said on Monday it would give awards to the border guards who rammed a Ukrainian naval tugboat south of Kerch before capturing it along with two artillery boats.

The two dozen Ukrainian sailors captured in the incident have been moved to Moscow and face up to six years in prison on charges of violating Russia's borders.