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

The United Nations Security Council scheduled an emergency meeting for Monday to discuss the abrupt escalation in tensions. The episode also threatened to shake up Ukraine’s fragile internal politics.

At a midnight meeting, Ukraine’s National Security and Defense Council said it would ask Parliament to declare 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.

“This whole story grows more complicated by the fact that during martial law, it is forbidden to hold presidential, parliamentary or local elections, as well as strikes, protests, rallies and mass actions,” Mustafa Nayyem, a member of Parliament, posted on Facebook.

The Ukrainian Navy left little ambiguity in asserting that its ships had been attacked. “Fighter jets used weapons against the naval ships of the armed forces of Ukraine,” the navy’s statement said.