Russia on Sunday opened fire at three Ukrainian naval vessels attempting to cross the Kerch Strait separating the Black Sea and the Sea of ​​Azov, in a major flare up of the conflict between the two countries.

The Ukrainian Navy said Russian border guards fired at its ships in waters off the Crimean Peninsula and authorities in Kiev released a statement saying two people were injured in the incident.

News of the escalation came just hours after the Ukrainian Navy said that a ship belonging to the Russian border guard rammed one of its tugboats.

Russia's Federal Security Service - known as the FSB - said the three ships were seized and three Ukrainian naval personnel received medical assistance for non-life-threatening injuries, according to the state-run TASS news agency.

The FSB said it had "irrefutable proof" that Kiev was using its navy "to stage a provocation in the Black Sea," and added that "weapons were used to force the Ukrainian warships to stop" as they had been performing "dangerous manoeuvres."

According to a statement from authorities cited by Russian media, Ukraine had sought "to create a conflict situation."

Ukraine alleges that Russia is in violation of the UN Convention on the Law of the Sea and of a contract between Kiev and Moscow on the use of the Sea of Azov and the Kerch Strait.

Moscow in May inaugurated a bridge across the Strait, linking the annexed Crimean Peninsula with Russia. The Kerch Strait is the only way into the Sea of Azov.

Ukrainian President Petro Poroshenko said the parliament will on Monday hold a special session to decide whether martial law should be imposed.

Following an emergency meeting of the National Security and Defence Council to discuss the escalation, Poroshenko also said the military reserve would be made ready.

This, however, did not mean an immediate mobilization, Poroshenko said, according to Russian news agency Interfax.

US ambassador to the United Nations Nikki Haley said an emergency session of the Security Council will be held to discuss the situation.

The meeting will take place Monday 11 am (1600 GMT), Haley tweeted.

The EU and NATO called on both sides to exercise 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," EU foreign affairs spokeswoman Maja Kocijancic said in a statement.

NATO spokeswoman Oana Lungescu called for "restraint and deescalation" and said NATO was in contact with Ukrainian authorities

"NATO‎ fully supports Ukraine's sovereignty and its territorial integrity, including its navigational rights in its territorial waters," Lungescu said.

"We call on Russia to ensure unhindered access to Ukrainian ports in the Azov Sea, in accordance with international law," she added.

The Sea of Azov, north-east of the Crimean Peninsula, has become a new arena in the conflict between Ukraine and Russia in recent months.

Russia annexed the Crimea region, the site of a major Russian naval base on the Black Sea, in 2014 in retaliation for Ukraine ousting its pro-Russian president, amid mass protests calling for closer ties with the West.

Fighting then broke out in eastern Ukraine, with the West accusing Russia of supporting pro-Moscow separatists in the region.

More than 10,000 people have since been killed in the conflict, according to estimates by the United Nations.