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 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.