Russias 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.
Mr. Poroshenko delivered a speech to Ukraines Parliament asking it to approve the declaration of martial law starting on Wednesday, with the military already on full alert. The attack on the naval vessels near the shared waterway, the Kerch Strait, represented a new stage of aggression in what he called Russias hybrid war against Ukraine.
This is a bold and frank participation of the regular units of the Russian Federation, their demonstrative attack on the detachment of the Ukrainian Armed Forces, said Mr. Poroshenko, This is a qualitatively different situation, a qualitatively different threat.
Members of 450-member Verkhovna Rada, the Parliament, who were present voted overwhelmingly to support the measure — 276 to 30 — after the president agreed to dilute its scope.
Ukraine also received a boost from the international reaction, underscoring both the isolation of Russia from the West over the Ukraine conflict, and the desire to protect the international maritime convention that allows for unimpeded shipping through any strait.
What you saw yesterday was very serious, because you saw actually that Russia used military force in an open way, said NATOs secretary general, Jens Stoltenberg, during a news conference in Brussels following a meeting requested by Ukraine. This is escalating the situation in the region and confirms a pattern of behavior which we have seen over several years.
NATO was increasing its military presence in the area, he said, calling on Russia to allow freedom of navigation for Ukrainian ships in the Kerch Strait.
At the United Nations, Russia called a session of the Security Council in an attempt to force a discussion about what it called Ukrainian violations of Russian territorial waters. But Western nations quickly turned the session into a long criticism of Russia for its actions against Ukraine since 2014.
Impeding Ukraines lawful transit through the Kerch Strait is a violation under international law. It is an arrogant act that the international community must condemn and will never accept, Ambassador Nikki R. Haley of the United States told the council.
As President Trump said many times, the United States would welcome a normal relationship with Russia, but outlaw actions like this one continue to make that impossible, she said.
Mr. Trump offered his own criticism — without specifically blaming Russia — when asked by reporters in Washington about the naval confrontation. Not good. Were not happy about it at all, he said. We do not like whats happening either way.
Various European capitals also criticized Russia, calling for it to release the seized vessels and their crews. There were scattered calls for new sanctions against Russia.
The Russians seemed to try to tamp down the confrontation, moving an old cargo vessel anchored to block passage through the Kerch Strait, and allowing commercial traffic to resume.
The Kremlin remained largely silent for much of the day. It was left to Foreign Minister Sergey V. Lavrov to address the issue, and his ministry accused Ukraine of creating threats to normal shipping traffic in the strait by violating international maritime law, and trying to foment a crisis for domestic political purposes.
Russian political analysts suggested that the Kremlin had no need to ratchet up its confrontation with Ukraine — it had already achieved what it wanted by destabilizing the country through support for the separatists.
While President Vladimir V. Putins flagging domestic support may benefit from a fight with Ukraine, that would be outweighed by the risk of greater Western sanctions. I do not see any benefits for the Kremlin from this confrontation, said Nikolai Petrov, an analyst and professor of political science.
Asked about events during his daily briefing, Dmitri S. Peskov, Mr. Putins spokesman, framed the Russian actions against the Ukrainian boats as an interception, not an attack.
The question here is of incursion into the territorial waters of the Russian Federation by foreign military vessels, Mr. Peskov said. They entered the territorial waters of Russia without responding to any queries from our border guards, in no way responded to offers to make use of pilotage service, and so on and so forth.
The dispute over the waterway is fundamentally unresolvable because it hinges on different interpretations of who controls the territorial waters around the Crimean peninsula. Both sides tried to portray themselves as determined to protect the normal shipping that the other side was interfering with. Ukraine had previously sought and been granted permission for similar passages, according to official Russian accounts, but did not this time.
But Ukraine wants to assert its continued sovereignty in areas which Russia considers its own, analysts said. Controlling passage from the Black Sea through the Kerch Strait into the Sea of Azov is a key element in asserting Russias broader claim to Crimea.
Moscow clearly seeks to turn the Azov Sea into a Russian basin, and to use it to bring leverage to bear on Kiev, wrote Mark Galeotti, an expert on Russian intelligence services at the Institute of International Relations in Prague, on Twitter. It wants to demonstrate its capacity to act without having to worry about external constraint.
The two sides signed an agreement in 2003 to guarantee free passage through the strait, but in recent months have been harassing each others ships. The port of Mariupol and a couple others are important for the Ukrainian economy for exports of steel and grain, as well as for imports.
Steven Pifer, a former American ambassador to Ukraine, said that the Kremlin might be testing the level of support for Ukraine using the waterway. They can very easily back off, he said. But if they sense the reaction is weak, I think that they will continue the blockade.
Earlier this year Mr. Putin inaugurated a $7.5 billion bridge across the strait, meant not least to assert its claim to Crimea with a physical link.
One strategic aspect of the design revealed by events on Sunday is that Russia could block the strait merely by anchoring a cargo ship in the one opening under the bridge — 185 meters wide and 35 meters high — large enough to allow the passage of ships.
President Poroshenko sought to portray events as part of a larger assault by Russia. During his speech to Parliament, he waved a sheaf of papers that he said detailed Ukraines intelligence about Russian preparations for a ground offensive. That seemed unlikely, given the lack of any clear military objective, analysts said.
There was, however, a widespread sense among opposition figures and analysts that Mr. Poroshenko aimed to put off the March election, noting that he had not called for martial law during previous points in the conflict when the fighting was far worse.
Mr. Poroshenko tried to assuage that criticism by cutting the period of martial law from two months to one, so it would not interfere with the official start of the campaign season on Dec. 31.
Other compromises mean that the martial law declaration will only affect the 10 provinces bordering Russia or Transnistria, a breakaway province of neighboring Moldova, also controlled by Russian-backed forces.
The president also promised that martial law would not be used to curb civil liberties or to announce a general military mobilization, and that it would only be enforced in the case of new attacks. Still, the very prospect of martial law could help boost support for him as a wartime leader.
Oleg Kashin, a Russian columnist and political analyst, wrote in the online publication Republic that the expansion of the shooting into the Sea of Azov seemed more like an extension of the endless skirmishing in eastern Ukraine than the start of any full-fledged war.
The Sea of Azov is the most convenient space for the most spectacular political wrestling, he wrote, calling it a tiny reservoir that nobody had ever considered a real sea.
He wrote that the sea belongs only to Russia and Ukraine, and no third-party interests will be affected, even if tomorrow the entire surface of the Sea of Azov goes up in flames.
Reporting was contributed by Andrew Kramer and Ivan Nechepurenko from Moscow, Iulia Mendel from Kiev, Ukraine, Milan Schreuer from Brussels and Michael Schwirtz and Rick Gladstone from New York.