Chancellor Angela Merkel has called for moderation in the flare-up between Russia and Ukraine, as Moscow decides whether to release more than 20 Ukrainian sailors it detained in the waters off Crimea.

Moscow shot at and seized three Ukrainian naval vessels near the disputed Crimea region on the weekend. The European Union, the US and NATO have called for the sailors and their vessels to be released immediately.

Merkel telephoned Russian President Vladimir Putin late Monday after earlier speaking to Ukrainian President Petro Poroshenko, also by telephone.

"The chancellor stressed the necessity for de-escalation and dialogue," government spokesman Steffen Seibert said after the call with Putin.

Ukraine's navy said Russian border guards fired at its ships in waters separating the Black Sea and the Sea of ​​Azov off the Crimean peninsula on Sunday.

Russia annexed Crimea, the site of a major Russian naval base, in 2014 in retaliation for Ukraine ousting its pro-Russian president, amid mass protests calling for closer ties with the West.

Ukrainian lawmakers approved a decree from Poroshenko on Wednesday that declared martial law in the area of Ukraine bordering Russia and the Black Sea coast for 30 days.

If the situation normalizes, martial law can be lifted again at any time, the head of the National Security Council Oleksandr Turchynov said.