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 the introduction of martial law, 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.
Sailors to appear in court
Moscow has so far resisted calls to release the three ships or the 24 sailors it has detained.
Emine Avamilyeva, a lawyer for one of the sailors, told AFP that 21 of them were expected to appear later Tuesday before a court in Simferopol, the main city in Russian-annexed Crimea.
Three others were wounded in the weekend clash and were being treated in a Crimean hospital, the lawyer said.
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 the Interfax Ukraine news agency. "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.
Kiev urged the United States and European Union to impose more sanctions on Russia over the latest incident.
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".
Pro-Kremlin newspaper Izvestia on Tuesday denounced the reaction as "predictably anti-Russian".
The United Nations 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" and slammed "yet another reckless Russian escalation".
She did not, however, threaten further sanctions on Russia and President Trump suggested it was up to European governments to handle the crisis.
"We don't like what's happening and hopefully it will get straightened out. I know Europe is not -- they are not thrilled. They're working on it too. We're all working on it together," Trump told reporters at the White House.
Trump and Putin are expected to meet later this week on the sidelines of the G20 summit in Argentina.