iStock/Thinkstock(NEW YORK) — Russia fired on and then seized three Ukrainian navy ships in the Black Sea close to Crimea on Sunday, injuring six crew members, according to Ukraine, marking a sharp spike in tensions between the two countries.

In a statement posted on its Facebook page, Ukraine’s navy said that ships belonging to Russia’s FSB border service had opened fire on two small frigates and a tug accompanying them. It said the Russian fire had immobilized the two frigates and that Russian special forces had then boarded and seized them, injuring six Ukrainian sailors.

Ukraine President Petro Poroshenko has called an emergency meeting of his military cabinet and ordered his foreign ministry to contact the United Nations Security Council, his spokesman said.

Russia’s FSB confirmed that it had seized the three Ukrainian ships and had used weapons to take them. In a statement, the agency said three Ukrainian sailors had been hurt, but that their injuries were not life-threatening and that they were receiving treatment.

The FSB said it had seized three ships because they had refused demands to stop and had unlawfully entered Russian waters as they tried to cross through the Kerch Strait, a narrow stretch of water separating the Russian mainland from Crimea, which Moscow seized from Ukraine in 2014.

The waters around Kerch have become the site of an increasingly sharp confrontation between Russia and Ukraine, with Ukraine accusing Moscow of blocking its ships’ access. The dispute has become an increasingly tense flashpoint between Russia and Ukraine in their larger conflict that was set off by the annexation of Crimea and Sunday’s incident has set off alarms that it could lead to new escalation.

The European Union and NATO on Sunday called for Russia to immediately stop blocking access through the Kerch Strait and urged both sides to show restraint.

“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 EU said in a statement.

Sunday’s incident began in the morning local time. Ukraine’s navy said its three ships, the Berdyansk, the Nikopol and the Yani Kapu, had been making a planned relocation from the city of Odessa to Mariupol, a port on the Sea of Azov that is under Ukrainian control and is located close to the frontline with rebels backed by Russia

As they headed towards the straits, the Ukrainian ships were met by Russian coast guard vessels. Photographs released by the FSB’s Crimean branch showed the Russian ships cutting in front of and sailing close to the Ukrainian boats.

Ukraine then accused one of the Russian ships of ramming the tug accompanying the two warships, damaging its engine and a lifeboat. As the incident continued, Russia blocked access to the Kerch Strait by parking a tanker in it.

The Kerch Strait is the only way into the Sea of Azov and a 2003 treaty defines the strait and the sea as shared territorial waters for Russia and Ukraine. However, since its seizure of Crimea, Russia has been exerting greater control over the straits.

The strait is spanned by a bridge recently constructed by Russia and the tanker’s presence now means that dozens of cargo ships waiting to pass through are now held up, The Associated Press reported.

Russia and Ukraine have been locked in conflict since Moscow’s annexation of Crimea and the subsequent war that followed in eastern Ukraine, where Russia has backed separatist rebels, covertly sending in troops to support them.

The war that has killed over 10,000 people continues in eastern Ukraine, but large-scale fighting has ceased since a peace agreement was reached in 2015. Almost daily shelling nonetheless continues along the frontline and Ukraine accused Russia of deploying regular troops there, as well as supplying the rebels with large amounts of weapons.

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