Russian President Vladimir Putin warned Ukraine against any "reckless acts" on Tuesday after Kiev declared martial law in response to Moscow's seizure of three of its navy vessels.
The Ukrainian parliament late on Monday voted in favor of President Petro Poroshenko's request for the introduction of martial law in parts of the country for 30 days.
Read more: Ukrainian president decrees martial law amid Russia standoff
The decision came as Ukraine and Russia face their most dangerous crisis in years after Russian forces fired on, boarded and captured Kiev's ships on Sunday off the coast of Crimea.
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.
Ukrainian Defense Minister Stepan Poltorak (C) speaks with lawmakers before a vote to introduce martial law, after Russia seized Ukrainian naval ships off the coast of Russia-annexed Crimea, during an Ukrainian Parliament session in Kiev, November 26, 2018. /VCG Photo
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 and offers of mediation.
Martial law gives Ukrainian authorities the power to mobilize citizens with military experience, regulate the media and restrict public rallies in affected areas.
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.
In a phone conversation with German Chancellor Angela Merkel, Putin expressed "serious concern" over the introduction of martial law, the Kremlin said in a statement.
Putin said Kiev's actions were "clearly taken in view of the election campaign in Ukraine".
He said he hoped Berlin could intervene with Ukrainian authorities "to dissuade them from further reckless acts".