Russia’s Federal Security Service, known as the FSB and which oversees the coast guard, said there was “irrefutable evidence that Kiev prepared and orchestrated provocations ... in the Black Sea. These materials will soon be made public.”

The FSB confirmed early Monday that it fired on the vessels to force them to stop, and then seized them.

The European Union and NATO called for restraint from both sides and for Moscow to restore access to the strait, which Ukraine uses to move ships to and from ports on either side of the peninsula.

Ukrainian authorities said they had given advance notice to the Russians that the vessels would be moving through the strait, which connects the Black Sea to the Sea of Azov.

Russia said the three Ukrainian vessels made an unauthorized passage through Russian territorial waters, while Ukraine alleged that one of its boats was rammed by a Russian coast guard vessel.

The tugboat, which was rammed, was traveling with the two Ukrainian gunboats from Odessa on the Black Sea to Mariupol, an eastern Ukraine port, via the Kerch Strait.

“Russian coast guard vessels ... carried out openly aggressive actions against Ukrainian navy ships,” the Ukrainian statement said, with the tugboat suffering damage to its engine, hull, side railing and a lifeboat.

The Kerch Strait is the only passage into the Sea of Azov. The strait is spanned by the recently completed Kerch Bridge, connecting Crimea to Russia. Transit under the bridge has been blocked by a tanker ship, and dozens of cargo ships awaiting passage are stuck.

Russia has not given any indication of how long it will block the strait, but a long-term closure to would amount to an economic blockade of Ukrainian cities on the Azov coast. Russia’s Black Sea Fleet greatly outmatches the Ukrainian navy.

Ukrainian ports on the Sea of Azov include strategically vital centers such as Mariupol, the closest government-controlled city to Donetsk and Luhansk, the breakaway regions of eastern Ukraine controlled by Russia-backed separatists. Thousands of people in those regions have been killed in fighting between Ukrainian troops and the separatists since 2014.

The FSB told Russian news agencies after the first incident that the Ukrainian ships held their course and violated Russian territorial waters.

“Their goal is clear — to create a conflict situation in the region,” the FSB statement said. It did not mention that a Ukrainian tugboat was rammed.

Although a 2003 treaty designates the Kerch Strait and Sea of Azov as shared territorial waters, Russia has been asserting greater control over the passage since 2015.

The Ukrainian Foreign Ministry said earlier that Russia’s actions violated the UN Charter and international law, and pledged to “promptly inform our partners about Russia’s aggressive actions.”

“Such actions pose a threat to the security of all states in the Black Sea region,” the statement said, “and therefore require a clear response from the international community.”

Ukrainian President Petro Poroshenko, after a meeting with his National Security Council, said in a statement he would ask parliament Monday to take up the issue of whether to implement martial law over the incident.

About 50-100 people gathered outside the Russian Embassy in Kiev to protest Moscow’s actions.

Dmitry Kiselyov, a commentator on Russian state-controlled TV, told viewers of his Sunday evening news program that Poroshenko, encouraged by the US, is looking to pick a fight with Russia in the Black Sea.

The talk show host also alleged the US talked Poroshenko into staging a provocation against Russia as a means to disrupt an upcoming meeting between Presidents Vladimir Putin and Donald Trump at this week’s Group of 20 summit in Argentina.

“What is happening now at the (Kerch) bridge threatens to turn into a very unpleasant story,” Kiselyov said.