Ukraine will not allow Russian men aged 16-60 into the country following the imposition of martial law, Kiev says.
An exception would be made for “humanitarian cases” such as those travelling to funerals. Russia says it is not planning retaliatory measures.
Martial law has been imposed in 10 Ukrainian regions until 26 December.
The move came amid fears of a Russian invasion after Russian forces seized three Ukrainian vessels and 24 sailors in the Black Sea last Sunday.
Ukraine said the incident was a flagrant violation of international law, while Russia says the vessels violated its territorial waters.
It is the most dangerous clash at sea off Crimea since Russia annexed the Ukrainian peninsula in March 2014.
