Russia has seized three Ukrainian naval ships in waters near Moscow-annexed Crimea, raising fears of military escalation and prompting an emergency meeting of the UN Security Council on Monday.

In an unprecedented incident, Russia used weapons against the Ukranian ships which it claims illegally entered its waters before boarding and searching them, the country’s FSB security service confirmed.

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

Ukraine said a Russian border guard vessel rammed the tugboat in “openly aggressive actions” and then fired on the ships, immobilizing all three.

It said the Kerch Strait was blocked by a tanker and that Russian military aircraft were flying over the area.

The confrontation at sea is a dangerous development in the long-running conflict pitting Ukraine against Moscow and Russian-backed rebels in the east of the country.

The UN Security Council will hold an emergency meeting on Monday to discuss the escalation, US ambassador to the UN Nikki Haley said. The meeting was requested by both Russia and Ukraine, diplomatic sources told AFP.

Meanwhile, Ukrainian President Petro Poroshenko has asked parliament to vote Monday on whether to impose martial law on the country for 60 days, after his Military Cabinet recommended the move in a late-night meeting.

Throwing his weight behind the measure, which is not guaranteed to pass, he said it “in no way means that Ukraine will carry out any offensive actions”.

“I want to emphasize separately that we have all irrefutable evidence that this aggression, this attack on the Ukrainian Navy’s warships was not a mistake, not an accident, but a deliberate action,” he said in a statement.

Ukraine said six of its soldiers were injured, two seriously, but Russia’s FSB reported only three had suffered non-life threatening injuries and were given medical treatment.

Protesters, some armed with flares, rallied outside the Russian embassy in Kiev on Sunday night in response to the latest development.

The seizure has sparked alarm among Kiev’s Western allies.

“We expect Russia to restore freedom of passage at the Kerch strait and urge all to act with utmost restraint to de-escalate the situation immediately,” the European Union said in a statement.

NATO also urged “restraint and de-escalation”, and called on Russia “to ensure unhindered access to Ukrainian ports in the Azov Sea, in accordance with international law”.

Ukraine’s foreign ministry said the “aggressive actions” were a violation of international law that would be met with “an international and diplomatic legal response”.

Russia claims the waters off Crimea after annexing the peninsula in 2014.

“Having breached this morning the Russian border, the three Ukrainian ships — the Berdyansk, the Nikopol and the Yani Kapu — tried again on November 25 at 19:00 (Moscow time) to conduct illegal actions in Russian territorial waters,” the FSB said on Sunday.

The FSB, which oversees the border guard service, also said the Ukrainian ships had failed to respond to the “legal demands” of the Russian authorities.

Russian politicians accused Ukraine of acting at the behest of its Western allies and deliberately attempting to escalate the conflict.

Crimean leader Sergei Aksyonov wrote on Facebook that he “was sure the Western masters of the Kiev regime are behind this act of provocation”.

Pyotr Tolstoy, deputy speaker of the Russian parliament, warned on Facebook that “the Ukrainian puppet authorities… are risking unleashing a large military conflict”.

“One thing is clear: Russia won’t allow military provocations in its territorial waters.”

Ukraine said it gave Russia advance warning of its ships’ route, which its vessels are obliged to take to reach the Sea of Azov.

In September, the Ukrainian navy complained about “acts of provocation” by Russian border guards against its ships taking the same route.

Ukraine has increased the number of navy ships and border guard patrols in the Sea of Azov after Russia deployed five more warships.

Kiev says its naval buildup is due to Russia stepping up controls in the area this year and carrying out lengthy checks on commercial shipping that were very costly for freight and port companies.

Kiev and the West have accused Moscow of deliberately blocking ships from accessing Mariupol, which has vital access to heavy industry in the region.

Mariupol is close to the region of eastern Ukraine controlled by Russian-backed separatists in a conflict that has caused at least 10,000 deaths since 2014.

The United States, a key ally of Ukraine, is opposed to Russia’s takeover of Crimea and is looking to reduce Ukraine’s dependency on Russian gas pipelines.