An emergency meeting of the UN Security Council has been called after Russia seized three Ukrainian ships in the Black Sea.
Tensions between the two countries increased on Sunday in the Kerch Strait near Crimea after Russia moved to block several Ukrainian vessels.
Russian ships fired on the Ukrainian ones in a bid to make them stop, injuring several sailors.
Moscow claims the vessels illegally crossed the Russian border, adding that a criminal case has been opened.
Kiev, meanwhile, said Russia had opened fire on its ships 'deliberately' and injured six sailors.
President Petro Poroshenko said two of the sailors were 'severely wounded'.
Officials in Ukraine say they are planning to introduce martial law for 60 days in the wake of the incident, although the proposal requires parliamentary approval.
Mr Poroshenko said: "We have irrefutable evidence that this aggression, this attack on the Ukrainian Navy’s warships was not a mistake, not an accident, but a deliberate action... including the use of weapons against Ukrainian sailors.
"Ukraine will carry out its actions exclusively for defense of its territory, protection and safety of its citizens."
In a statement quoted by state media, Russia's security service (FSB) said: "The Ukrainian Navy’s vessels the Berdyansk, the Nikopol and the Yany Kapu, which violated Russia’s state border this morning, made another attempt of committing illegal activities in Russia’s territorial sea at 19:00 Moscow time on November 25.
"They did not respond to legitimate demands by the ships and boats of Russia’s FSB Border Guard Service escorting them to stop immediately and performed dangerous maneuvers."
While tankers had initially been used to block access to the Kerch Strait, Russian state media confirmed access had now been reopened for civilian ships.
Nikki Haley, the US ambassador to the UN, confirmed the Security Council will convene on Monday over the situation.
An emergency Security Council meeting has been called for tomorrow at 11:00am https://t.co/FpCAD2rGPn
— Nikki Haley (@nikkihaley) November 26, 2018
The latest developments represent a further worsening of the relationship between Kiev and Moscow.
Relations have been particularly fraught since Russia annexed Crimea in 2014.
