MOSCOW (AP) — Ukraine’s president demanded Monday that Russia immediately release Ukrainian sailors and ships seized in a standoff around Crimea that sharply escalated tensions between the two countries and drew international concern.

The two neighbors have been locked in a tense tug-of-war since Russia’s 2014 annexation of Crimea, but the incident late Sunday in which Russian coast guard ships fired on Ukrainian navy vessels near the Kerch Strait directly pitted the two militaries, placing them on the verge of an open conflict.

Ukrainian lawmakers were set to consider a presidential request for the introduction of martial law in the country later on Monday.

The Ukrainian navy said six of its seamen were wounded when Russian coast guards opened fire on three Ukrainian ships near the Kerch Strait and then seized them.

Russia’s Federal Security Service that is in charge of the coast guard said that three Ukrainian sailors were lightly injured and given medical assistance. It said the Ukrainian boats were towed to the nearby port of Kerch.

An emergency meeting of the United Nations Security Council was also called for Monday. The European Union and NATO called for restraint from both sides.

Ukrainian President Petro Poroshenko said at a meeting of Ukraine’s national security council Monday that “we demand that (the ships and crews) are urgently turned over to the Ukrainian side” and called for a “de-escalation” of the crisis around Crimea.

Poroshenko had a phone call Monday with NATO Secretary-General Jens Stoltenberg to discuss the situation. NATO later said that at Poroshenko’s request its ambassadors and Ukraine’s envoy will hold emergency talks in Brussels later Monday.

NATO said Stoltenberg expressed the U.S.-led military alliance’s “full support for Ukraine’s territorial integrity and sovereignty, including its full navigational rights in its territorial waters under international law.”

Russia and Ukraine have traded blame over the incident that further escalated tensions that have soared since Russia annexed Ukraine’s Crimean Peninsula in 2014 and backed a separatist insurgency in eastern Ukraine.

Ukraine said its vessels were heading to the Sea of Azov in line with international maritime rules, while Russia charged that they had failed to obtain permission to pass through the Kerch Strait separating Crimea from the Russian mainland.

The narrow strait is the only passage between the Black Sea and the Sea of Azov. It’s spanned by a 19-kilometer (11.8-mile) bridge that Russia completed this year.

“There is no doubt that it was done by blessing or, perhaps, even a direct order from the top,” said Russian Foreign Minister Sergey Lavrov. “While planning that provocation, Ukraine had undoubtedly hoped to get additional benefits from the situation, expecting the U.S. and Europe to blindly take the provocateurs’ side.”

He urged the West to “calm down those in Ukraine who are trying to unleash a military hysteria to get political gains in connection with the planned elections” - a reference to Ukraine’s presidential vote in March.

Hundreds of protesters from far-right party National Corps waved flares at a protest in the snowy streets outside the Ukrainian parliament Monday. They brandished yellow-and-blue flags with the Ukrainian national symbol, the trident, and a huge white banner reading ‘Don’t back down!”

Poroshenko chaired an emergency meeting of his Cabinet early Monday and asked parliament to introduce martial law in response to what he described as Russian aggression.

“We consider it as an act of aggression against our state and a very serious threat,” the president said. “Unfortunately, there are no ‘red lines’ for the Russian Federation.”

Russia closed the Kerch Strait for sea traffic Sunday by positioning a tanker under the bridge spanning it. It reopened the route early Monday.

The seizure of the Ukrainian ships followed a tense situation in which the three Ukrainian vessels were maneuvering near the Kerch Strait for hours shadowed by Russian coast guard boats.

The incident came after months of tensions and incidents in the Sea of Azov that involved inspections and seizures of ships.

While a 2003 treaty designates the Kerch Strait and Sea of Azov as shared territorial waters, Russia has sought to assert greater control over the passage since the annexation of Crimea.

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Yuras Karmanau in Minsk, Belarus, contributed to this report.