Russian President Vladimir Putin on Tuesday warned Ukraine against any "reckless acts" after Kiev declared martial law in response to Moscow's seizure of three of its navy vessels.
The Ukrainian parliament late Monday voted in favour of President Petro Poroshenko's request for the introduction of martial law in border areas for 30 days.
The move came after Russian forces fired on, boarded and captured three of Kiev's ships on Sunday off the coast of Crimea, sparking the most dangerous crisis between the ex-Soviet neighbours in years.
The incident was the first major confrontation at sea in the long-running conflict pitting Ukraine against Moscow and Russian-backed separatists in the country's east.
It has raised fears of a wider escalation - in a conflict that has killed more than 10,000 people since 2014 - and prompted international calls for restraint.
Martial law gives Ukrainian authorities the power to mobilise citizens with military experience, regulate the media and restrict public rallies in affected areas.
In a phone conversation with German Chancellor Angela Merkel, Putin expressed "serious concern" over its introduction, the Kremlin said in a statement.
He said he hoped Berlin could intervene with Ukrainian authorities "to dissuade them from further reckless acts".
Moscow has accused Kiev of planning Sunday's confrontation as a provocation aimed at drumming up support for Poroshenko ahead of elections next year and convincing Western governments to impose further sanctions on Russia.
Putin said Kiev's actions were "clearly taken in view of the election campaign in Ukraine".
Kremlin spokesman Dmitry Peskov told reporters Tuesday that Kiev's martial law threatened to cause an "escalation of tensions in the conflict region" in the east of the country.
Moscow has so far resisted calls to release the three ships or the 24 sailors it has detained.
Some of the sailors will face trial in Simferopol - the main city in Russian-annexed Crimea - on Tuesday, the peninsula's human rights ombudsman Lyudmila Lubina told AFP.
The rest are expected to face trial on Wednesday, she said, while three others were still in hospital after being wounded in the weekend clash.
Moscow accuses them of crossing illegally into Russian waters and of ignoring warnings from its border guards, with officials suggesting they could face criminal prosecution.
Sunday's incident has been playing out on Russian and Ukrainian television screens, with dramatic footage of Russian ships chasing down a Ukrainian tugboat that was trying to pass through the Kerch Strait from the Black Sea into the Sea of Azov.
Russian state television late on Monday aired footage of some of the captured sailors being questioned by Moscow's security services.
One of the sailors is heard saying "the actions of the Ukrainian armed vessels in the Kerch Strait had a provocatory character" - parroting the version of events put forward by Russian authorities.
Ukraine's naval commander, Igor Voronchenko, said the sailors were pressured into giving false evidence.
"I know these sailors, they were always professional.
What they are saying now is not true," he told Ukrainian media.
"They (the Russians) could even say that we came from the sky on a spaceship."
Ukraine has accused Russian border patrol vessels of ramming the tugboat, which was accompanied by two small warships, and of firing on the Ukrainian vessels.
Western governments have rallied behind Kiev in the dispute, accusing Russia of illegally blocking access to the Sea of Azov and of taking military action without justification.
Britain, Canada, France, Germany and others expressed support for Kiev on Monday, with EU President Donald Tusk calling for Russia to return the Ukrainian sailors and ships and "refrain from further provocations".
The foreign minister of Austria, which holds the rotating EU presidency, said Tuesday that the EU will next month consider further sanctions against Moscow over the flare-up.
Pro-Kremlin newspaper Izvestia on Tuesday denounced the reaction as "predictably anti-Russian".
The UN Security Council met in an emergency session on the crisis on Monday, where US envoy Nikki Haley called the seizure of the ships an "outrageous violation of sovereign Ukrainian territory."