Ukraine has deployed troops to the border in preparation to counter what it said to be a 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 occupied Crimea”.
The day before, Mr Poroshenko claimed that Russia was trying to capture the major port city of Mariupol and create a land corridor to Crimea.
Vladimir Putin's spokesman yesterday called this statement “absolutely absurd” 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 enter the Azov Sea through the Kerch Strait on November 25.
Mr Putin opened a bridge over the strait between Crimea and mainland Russia in May.
Two dozen captive Ukrainians sailors face up to six years in prison on charges of violating Russia's borders.
Moscow has been backing separatists in a conflict in eastern Ukraine that has killed more than 10,000 people since 2014.
The Ukrainian military said yesterday it had begun calling up its first line of active reserves, which include men up to age 40 who have previously undergone armed forces training, as well as cadets.
It promised the men would be home in time for in time for the holidays.
The armed forces are also planning military exercises in two regions near Crimea, which Moscow annexed after protests forced out a Russia-friendly government in Kiev in 2014.
Meanwhile, Russia has moved anti-ship missiles toward the Kerch Strait and deployed an additional battery of S-400 surface-to-air missiles.
This weekend, Mr Poroshenko called for talks between the Normandy Four—France, Germany, Russia and Ukraine—to de-escalate the situation.
Kiev has accused authorities in the Kerch Strait of a “blockade” against Ukrainian ports on the Azov Sea.
More than 60 large ships were stopped south of the strait when The Telegraph visited for two days last week, but the owners of one said it had been delayed by inclement weather.