Moscow and Kiev have traded angry accusations since Russian navy vessels fired on, boarded and captured the three Ukrainian ships off the Ukrainian peninsula of Crimea which was annexed by Russia in 2014.
Ukraine's president has said that the country has barred Russian men between 16 and 60 from traveling to the country.
The move comes as the long-simmering conflict between the two nations escalated in the Black Sea on Sunday. Russian border guards opened fire on and captured three Ukrainian vessels and their 24-member crew off the Ukrainian peninsula of Crimea, which was annexed by Russia in 2014.
Francis Collings explains how Kiev is trying to tackle this.
President Petro Poroshenko tweeted on Friday that the restrictions on Russian travelers have been taken in order to prevent the Russians from forming "private armies" fighting on Ukrainian soil.
The head of the border service, Petro Tsygykal, said in a televised meeting on Friday that the entry of “foreigners” had been limited, primarily for citizens of the Russian Federation, more specifically mentioning the “non-admission of citizens of the Russian Federation aged from 16 to 60, male.”
After warning of the threat of "full-scale war", Poroshenko on Wednesday signed an act imposing martial law for 30 days in regions bordering Russia, the Black Sea and the Sea of Azov.
Kiev has demanded the return of its ships and the release of 24 sailors taken prisoner during the confrontation.
Tensions between Kiev and Moscow spilt over into confrontation when pro-EU protests in Ukraine led to the ousting of pro-Moscow president Viktor Yanukovych in 2014.
Russia annexed Crimea while pro-Moscow separatists seized parts of the Donetsk and Lugansk regions of eastern Ukraine, declaring unrecognised breakaway statelets in a conflict that remains unresolved.