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.

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 Defense Council said it would ask Parliament to declare a state of martial law. That raised alarms that President Petro O. 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 recognize.

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 U.N. Law of the Sea, which should guarantee access through the strait.

The New York Times

Andrew E. Kramer © 2018 The New York Times