MOSCOW — Ukraine’s president put his nation on a war footing with Russia on Monday, as tensions over a shared waterway escalated into a crisis that dragged in NATO and the United Nations.

Russia’s seizure a day earlier of three small Ukrainian naval vessels and 23 sailors — including at least three wounded in a shooting by the Russian side — was the first overt armed conflict between the two sides since the beginning days of the conflict in 2014, when Russian special forces occupied Crimea.

The opening of an additional front at sea, even if Ukraine lacks a real navy, introduced an unstable element into what had been a shadowy war. The conflict pitting Ukrainian soldiers against Russian-backed separatists in the breakaway Donbas region, in eastern Ukraine, has sputtered along for almost five years with more than 10,000 people killed.

The Kremlin, along with some Ukrainian opposition figures, called the martial drumbeats echoing from Kiev a domestic political ploy by its embattled president, Petro O. Poroshenko. They accused him of fearmongering in order to delay or at least reconfigure the March 31 election that he had seemed certain to lose.