Ukraine’s president has said Russian men between 16 and 60 have been barred from travelling 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.
Ukrainian president Petro Poroshenko tweeted that the restrictions on Russian travellers have been taken in order to prevent Russians from forming “private armies” fighting on Ukrainian soil.
On Monday, the Ukrainian parliament adopted the president’s motion to impose martial law in the country for 30 days in the wake of the naval stand-off near the Crimean Peninsula, which Moscow annexed in 2014.
The Ukrainian vessels had been trying to pass through the Kerch Strait on their way to the Sea of Azov.
The announcement follows Thursday’s decision by US president Donald Trump to scrap the much-anticipated meeting with Russian leader Vladimir Putin.
Mr Trump said it was not appropriate for him to meet Mr Putin since Russia has not released the Ukrainian seamen. A Russian government-appointed ombudswoman for Crimea told Russian news agencies that all the seamen have been transported from a detention centre in Crimea. The three commanders have been taken to Moscow, she said.
A Crimea court earlier this week ruled to keep the Ukrainian seamen behind bars for two months pending the investigation. There has been growing hostility between Ukraine and Russia since Moscow’s annexation of the Crimean Peninsula from Ukraine in 2014.
Russia has also supported separatists in Ukraine’s east, with clandestine dispatches of troops and weapons.
Fighting there has killed at least 10,000 people since 2014, but eased somewhat after a 2015 truce.
Meanwhile, Ukraine’s intelligence agency is searching the home of the father superior of Kiev’s biggest and oldest monastery, which is part of the Russian Orthodox Church. Officers are searching the home of Father Pavlo, who leads the Pechersk Monastery in Kiev.
Ihor Guskov, chief of staff of the SBU intelligence agency, told reporters that the cleric is suspected of “inciting hatred”.
The Ukrainian church, which has been part of the Russian Orthodox Church for centuries, moved close to forming an independent church – fuelled by the conflict with Russia Ukraine’s Orthodox communities earlier this year.
There are currently three Orthodox communities in Ukraine, including two breakaway churches.
Ukrainian authorities have sought to portray the Russian Orthodox clerics in Ukraine as supporting separatists.
