KIEV: Kiev and Moscow faced their worst crisis in years on Monday as Ukraine and its Western allies demanded the release of three ships fired on and seized by Russia near Crimea.
Russian forces boarded and captured the ships on Sunday, with Moscow accusing the vessels of illegally entering Russian waters off the coast of Crimea in the Sea of Azov.
Ukraine's military was on high alert and parliament was meeting to vote on a request to impose martial law from President Petro Poroshenko who accused Moscow of a "new phase of aggression."
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".
The confrontation is a 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.
"We will demand (at the Security Council) the immediate release of our sailors and the liberation of our ships."
In a televised address to the nation, Poroshenko accused Russia of taking the two countries' long-running conflict to another level.
"(Russia) entered a new phase of aggression," Poroshenko said, adding that the incident showed "the arrogant and open participation of regular units of Russian troops" after Moscow always insisted its forces were not directly involved in Ukraine.
Moscow blamed Kiev for the incident, with Kremlin spokesman Dmitry Peskov saying: "The Russian side acted strictly within both domestic and international law."
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.
Video purportedly showing a Russian vessel ramming a Ukrainian tug boat Sunday:
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.
"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.
"The incident marks a significant escalation of the conflict between Russia and Ukraine," research firm Eurasia Group said.
"Western governments will side with Ukraine against Russia over the incident... making new sanctions against Russia likely."
Protesters gathered in several Ukrainian cities to denounce Russia's actions, with several hundred far-right activists marching to parliament in Kiev and setting off flares.