Ukraine has banned Russian men aged between 16 and 60 from entering the country, the head of border service has said.
The move comes amid rising tensions between the two nations after the seizure of three Ukrainian naval ships and their 24-member crew in the Black Sea by Russia.
Announcing the ban, head of the border service Petro Tsygykal said in a televised meeting on Friday: "Today, the entry of foreigners is limited - primarily citizens of the Russian Federation - non-admission of citizens of the Russian Federation aged from 16 to 60, male."
Ukraine imposed martial law this week, citing fears that Russia was planning a full scale invasion.
Ukrainian president Petro Poroshenko tweeted on Friday that the restrictions on Russian travellers have been taken in order to prevent the Russians from forming "private armies" fighting on Ukrainian soil.
The president showed the broadcaster images of what he claims are tanks lining up along the border, close to where Russia stores its ammunition.
The long-simmering conflict between the two nations bubbled over on Sunday when Russian border guards opened fired on three Ukrainian vessels in the Black Sea.
The Russians then captured the crew.
Russia says the Ukrainians had violated its border while Ukraine says its ships were acting in line with international maritime rules.
