MOSCOW (NYTIMES) - A dispute between Ukraine and Russia in which each accuses the other of violating laws of the sea escalated sharply on Sunday (Nov 25) when the Ukrainian navy said the Russian military opened fire on several of its ships, wounding six sailors and seizing the vessels.

The incident in and around the Kerch Strait, a narrow body of water separating the Black and Azov seas, marked a pivot in the undeclared war, now nearly five years old, between the former members of the Soviet Union.

The Ukrainian military has been fighting two separatist movements in eastern Ukraine that are clearly backed by Moscow, though the Russian government has formally denied any direct military intervention.

That is in contrast to Sunday's events, which unfolded as a direct confrontation between Russia and Western-backed Ukrainian armed forces at sea.

At a midnight meeting, Ukraine's National Security and Defence Council said it would ask Parliament to declare a state of martial law.

That raised alarms that President Petro Poroshenko could use the incident to delay a presidential election scheduled for March that polls suggest he is unlikely to win.

The Ukrainian navy left little ambiguity in asserting that its ships had been attacked.

Russia's Federal Security Service, a law enforcement agency known as the FSB that oversees the coast guard, initially issued a statement saying the Ukrainian ships had, after an altercation, altered course to return to a Ukrainian port. But it later said Russia had detained the vessels and was providing first aid to three sailors.

The FSB said in a statement carried by Russian news agencies that the Ukrainian naval convoy of two small warships and a tugboat had entered Russian territorial waters near Crimea on Sunday in a "provocation".

The Russian coast guard responded by trying to escort them out of the area, it said.

After annexing the Crimean peninsula in 2014, Russia claimed territorial waters off the peninsula's coast that Ukraine does not recognise.

Ukraine claims a right to patrol in the entire Sea of Azov under a treaty with Russia designating the sea as shared territorial water.

Ukraine says Russia's actions have violated the treaty, as well as the UN Law of the Sea, which should guarantee access through the strait.