Ukraine and its Western allies say Russia illegally blocked the strait and acted against international law by grabbing the vessels and sailors.

The martial law gives Ukrainian authorities the power to mobilise citizens with military experience, regulate the media and restrict public rallies.

Amid fears of a Russian ground offensive, Poroshenko had wanted the law to cover the entire country but it was limited to border regions after opposition from lawmakers.

Before the vote, Poroshenko had accused Moscow of a "new phase of aggression."

With Ukraine's military on high alert, images of several captured sailors were broadcast on Russia's state-run channel Rossiya 24 showing interrogations by Moscow's security services.

At one point, one of the sailors is heard saying that "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.

The incident has raised fears of a wider military escalation. The UN Security Council met in an emergency session, where US Ambassador Nikki Haley warned Russia against "outlaw actions".

US President Donald Trump later weighed in on the incident.

"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.

- Dangerous development -

The confrontation is the latest dangerous development in the conflict pitting Ukraine against Moscow and Russian-backed rebels in the east of the country.

More than 10,000 people have been killed since the Moscow-backed insurgency broke out in April 2014 following Russia's annexation of Crimea from Ukraine.

The United States and EU have already imposed sanctions on Russia over the conflict and on Monday European capitals rallied behind Kiev.

Ukraine's Western allies accused Russia of using force without justification in the naval confrontation, while Kiev urged its partners to impose further sanctions on Moscow.

"These were planned acts of aggression by the Russian Federation against Ukraine," Foreign Minister Pavlo Klimkin told reporters in Kiev.

Poroshenko accused Russia of taking the two countries' long-running conflict to another level.

The incident showed "the arrogant and open participation of regular units of Russian troops," he said in a television address to the nation, disparaging Moscow's insistence that its forces were not directly involved in Ukraine.

Kremlin spokesman Dmitry Peskov blamed Kiev, saying: "The Russian side acted strictly within both domestic and international law."

- 'New phase' of conflict -

The crisis unfolded as two small Ukrainian warships and a tugboat were heading through the Kerch Strait, a waterway that gives access to the Sea of Azov from the Black Sea and which is used by both Ukraine and Russia.

Ukraine said a Russian border guard vessel rammed the tugboat and then fired on the ships, immobilising all three. It said the Kerch Strait was blocked by a tanker and that Russian military aircraft were flying over the area.

Russia's FSB security service, which oversees border forces, confirmed weapons had been fired and the vessels seized, accusing the Ukrainian ships of "violating the Russian border".

Ukraine's interior minister released a video on Twitter apparently from aboard one of the Russian ships, showing the tugboat being chased down and the collision, interspersed with commands and swearing in Russian.

Russian television networks showed a similar video, but with the voices removed and without the moment of collision.

Ukraine said six of its servicemen were injured in the incident, two seriously. The FSB said only three had suffered non-life threatening injuries and were given medical treatment.

Russian officials said 24 Ukrainian servicemen had been detained and Peskov said a criminal investigation had been opened.

- Europe rallies behind Kiev -

"I condemn Russian use of force in Azov Sea. Russian authorities must return Ukrainian sailors, vessels & refrain from further provocations," EU President Donald Tusk tweeted.

France also called on Russia to release the sailors and ships, with the foreign ministry saying "nothing appears to justify the use of force" by Russia.

German Foreign Minister Heiko Maas said any Russian "blockade" of the Sea of Azov was "unacceptable" and proposed French-German mediation to resolve the crisis.

Tensions have been building over the Kerch Strait, where Russia has built a new bridge that gives it a land connection to Crimea, annexed by Moscow in 2014.

Kiev has accused Moscow of blocking access for Ukrainian ships though the strait, the only way in and out of the Sea of Azov to the Black Sea.

In recent months both sides had deployed more naval and border vessels to the area. (AFP)