Moscow had prevented the ships from entering the strait by placing a large cargo vessel beneath a Russian-controlled bridge. Russia then closed the strait, which both nations use.

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Ukrainian President Petro Poroshenko endorsed the military cabinet’s proposal, rushed through in Kiev well past midnight, to install martial law across the country for 60 days. On Monday, the vote will be put before parliament and is expected to sail through.

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In Ukraine, martial law grants authorities and the military the powers needed to ensure national security. It could allow Poroshenko, who has been dogged by low ratings and could seek reelection in March, to act more independently than usual.

The mood in Kiev was bellicose. “It is now likely possible that Russia plans further acts of aggression at sea or on the ground. We must be ready for this,” Foreign Minister Pavlo Klimkin said on Twitter.

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Russia says the Ukrainian navy illegally entered its waters. The two artillery boats and a tugboat “unlawfully entered a temporarily closed area of Russia’s territorial sea at about 7 am Moscow time,” Russia’s border service told the Interfax news agency. “It is clear that their goal is to create a conflict situation in the region.”

Ukraine disputes this. The Ukrainian Foreign Ministry said in a statement that Russia acted “aggressively” and “illegally used force against the ships of the Ukrainian Navy.” A Russian ship rammed into the tugboat, damaging it, the Ukrainian navy added.

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A bilateral treaty grants both countries the right to use the Azov Sea.

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The standoff raises the specter of further confrontation between Russia and Ukraine. Both nations called for an emergency meeting of the U.N. Security Council. The European Union called on Russia to reopen the Kerch Strait and urged “all to act with utmost restraint to de-escalate the situation immediately.”

A lot is at stake. The two countries have been at loggerheads since a pro-Moscow government in Ukraine was toppled more than four years ago, touching off Russia’s annexation of Ukraine’s Crimean Peninsula and the ongoing war in eastern Ukraine involving pro-Russian separatists.

Fighting between the rebels and Ukrainian troops has claimed more than 10,300 lives since 2014, and continued skirmishes result in near-daily casualties.

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Maria Zakharova, a spokeswoman for Russia’s Foreign Ministry, said in a Facebook post that Ukraine was employing “the methods of banditry. First there are provocations, then they exert strong pressure, and finally an accusation of aggression.”

According to the Ukrainian navy, Russia sent two combat helicopters to the location of the incident.

The Ukrainian ships were on a journey through the Black Sea, having left the Ukrainian port city of Odessa and heading to Mariupol on the Azov Sea coast, the government-controlled Ukrainian city that is closest to the breakaway regions of Donetsk and Luhansk.

Since President Vladi­mir Putin earlier this year opened the new bridge across the Kerch Strait — connecting the Russian mainland to Crimea — Moscow has increased its control of the area.

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Tensions between Russia and Ukraine have been steadily mounting in recent months, and even entering new battlefields.

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Last month, the conflict spilled over into the area of faith when the Ukrainian Orthodox Church officially broke free from Moscow’s control, a move that continues to anger the Kremlin.

Amid the Black Sea standoff, popular Russian TV host Dmitry Kiselyov told state television that Poroshenko was picking a fight with Russia at the prompting of the United States.

“What is happening now at the [Kerch] bridge threatens to become a very unpleasant story,” said Kiselyov, one of the Kremlin’s main propagandists.