The Ukraine Navy says Russia fired on and seized three of its ships off the annexed territory of Crimea.

The alleged incident would be the most serious escalation of tensions to date between the two nations.

The EU, through its spokesperson Maja Kocijančič, urged both sides to “act with utmost restraint to de-escalate” the situation.

Two Ukranian gunboats and a tug were reportedly captured by Russian special forces. Two Ukranian crew members were injured in the incident.

EU call on Russia to allow Ukraine vessels freedom of passage through the area

Russia had accused the Ukranian ships of illegally entering its waters. Russian forces later blocked entry to the Kerch Straight by placing a cargo ship under a bridge.

The area is the only access to the Sea of Azov which is shared between the two nations.

The Russian Federal Security Service (FSB) blamed Ukraine for the incident and said “there is irrefutable evidence that Kiev prepared and orchestrated provocations … in the Black Sea.”

It promised to make public the evidence of Ukranian malfeasance.

Kocijančič said, however, that the EU expected Russia to “restore freedom of passage” through the Kerch Strait.

“The EU does not and will not recognise the illegal annexation of the Crimean peninsula by Russia,” she added.

Ukraine on high alert after its ships were hit

Ukrainian President Petro Poroshenko summoned his ‘war cabinet’ of top military and security advisors in the wake of the latest events.

Since Russia annexed the Crimean Peninsula in 2014 and claimed the neighbouring waters, tensions have run high in the area.

In September this year, the Ukranian Navy said that Russian border guards had committed “acts of provocation” against its ships in these waters.

Russia has also increased monitoring of commercial shipping in the area, prompting Ukraine to respond by increasing the number of naval vessels and border guard patrols it has stationed in the Sea of Azov.

Another source of tension has been Russia’s support for separatists forces in Ukraine.

Donald Trump has not directly responded to the latest incident, but he did return to one of his favourite themes on Twitter, that of European nations failing to contribute financially to NATO.

“Europe has to pay their fair share for Military Protection,” he tweeted, “The European Union, for many years, has taken advantage of us on Trade, and then they don’t live up to their Military commitment through NATO. Things must change fast!”

NATO issued a statement supporting Ukraine’s sovereignty

For its part, NATO issued a statement through Spokesperson Oana Lungescu, saying it “fully supports Ukraine’s sovereignty and its territorial integrity.”

It also echoed the calls from the EU for Russia to allow Ukraine unhindered access to its ports in the Azov Sea.

The three Ukranian ships involved were the Nikopol and Berdyansk gunboats and the Yana Kupa tug. Ukraine has said they were trying to sail from the Black Sea port of Odessa to Mariupol in the Sea of Azov. They were reportedly trying to leave the area when Russian forces fired on the gunboats and rammed the tug.