Ukraine's parliament is to decide whether to bring in martial law, after the capture of three of its naval vessels by Russia.
The three ships were sailing off the coast of Crimea, which was annexed by Russia in 2014, when they were seized.
Ukraine said Russia rammed one of its boats in an "act of aggression", while Russia said the ships had illegally entered its waters.
The incident marks a major escalation of tension between the two countries.
This is the first time the two militaries have come into open conflict in recent years, although Ukrainian forces have been fighting Russian-backed separatists in the east of the country.
The captured crew members are now being questioned in Kerch, Russia says.
What exactly happened?
On Sunday morning, Ukraine's Berdyansk and Nikopol gunboats, and the Yani Kapu tug, tried to sail from the Black Sea port of Odessa to Mariupol in the Sea of Azov.
Under a 2003 treaty between the governments in Moscow and Kiev, the Kerch Strait and the Sea of Azov are shared territorial waters.
However, Russia accused the Ukrainian ships of illegally entering its waters.
Ukraine says the Russians tried to intercept the ships, ramming the tug.
The vessels continued towards the Kerch Strait, the only way to enter the Sea of Azov, but were blocked by a tanker placed under the Kerch bridge. The ships were then seized by Russia.
Russia also scrambled two fighter jets and two helicopters to the area.
How do the Russian and Ukrainian accounts differ?
The Ukrainian navy said its boats were hit and disabled as they tried to leave the area. It said 23 Ukrainians were on board and six crew members had been injured.
Russia's FSB security service confirmed that one of its patrol boats had used force to seize the vessels but said only three sailors had been hurt.
Russia has also accused Ukraine of violating legal norms by sending the vessels into its territorial waters in a "pre-planned provocation".
Foreign ministry Sergei Lavrov said Ukraine broke "key provisions of international law" including naval law.
Under the current treaty, vessels from both sides enjoy freedom of navigation in the area.
However, Russia annexed Crimea, which borders the Ukrainian side of the strait, in 2014, following a Moscow-backed referendum that the UN condemned as illegal.
It also constructed the Kerch bridge linking mainland Russia to Crimea earlier this year, despite opposition from Ukraine.