Ukraine late on Monday adopted a bill that introduces martial law in several regions, including those bordering the separatist-held areas.
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, told reporters on Tuesday that martial law might trigger a flare-up in hostilities in the east.
Ukrainian troops have been fighting Russian-backed separatists in eastern Ukraine since 2014 but the hostilities have largely subsided since a truce was signed in 2015.
Germany's foreign minister is suggesting that Germany, France, Russia, and Ukraine could work together to resolve tensions between Moscow and Kiev following the weekend's incident near Russian-annexed Crimea.
Germany and France brokered a 2015 accord to end violence in eastern Ukraine, most of whose provisions remain unfulfilled. The two European powers have met at various levels with Russia and Ukraine over recent years in the so-called Normandy Format. Foreign ministry political directors from the four countries held a previously scheduled meeting Monday.
Foreign Minister Heiko Maas said Tuesday that Berlin has “called on Russia and Ukraine to show the greatest possible restraint, and offered to work on a solution in the Normandy Format.”
He added that “we must do everything for de-escalation, to prevent this crisis from turning into an even more serious crisis for security in Europe.”
Russia's main intelligence agency has released a video of three crewmembers of the Ukrainian vessels that were seized on Sunday.
The FSB put out the video on Tuesday, showing interviews with three seamen, all of whom confirmed that they violated the Russian border. It was not immediately possible to ascertain if the men were talking under duress. One of them was clearly reading from a teleprompter.
Russian border guards on Sunday fired at three Ukrainian navy vessels as they were trying to make their way through the Kerch Strait near Russian-occupied Crimea. The Russians then seized the ships and their crews.
The escalation over the weekend was the first overt military confrontation between Russia and Ukraine.
Russian President Vladimir Putin has told the German chancellor that he is ‘seriously concerned’ about the weekend's escalation near Crimea and developments in Ukraine.
Russian border guards on Sunday opened fire on three Ukrainian navy vessels and eventually seized them and their crews. Ukraine on Monday announced martial law in parts of the country, citing a "growing threat" from Russia.
The Kremlin said in the early hours Tuesday that Putin expressed his concern over the escalation and the martial law in Ukraine in a phone conversation with German Chancellor Angela Merkel. Putin told Merkel he lays all the blame for the confrontation on Ukraine and said Ukraine provoked it for political reasons.
Merkel's spokesman, Steffen Seibert, said Merkel in the conversation with Putin stressed the need for de-escalation and dialogue.