Appearing on state television, Poroshenko said there was a “serious threat” of a land invasion by Russia. He cited Ukrainian intelligence reports as saying Russian forces were forming just miles from the border with Ukraine.
Russia warned Ukraine of “serious consequences” if it continues what Moscow describes as stoking conflict.
Several Ukrainian sailors were injured in the Sunday morning skirmish, and 24 were detained by Moscow for allegedly provoking the incident near the Kerch Strait, a narrow strip of water separating the Black and Azov seas. Moscow has since closed off the Kerch Strait.
Martial law will take effect from 9 a.m. local time and will include a partial mobilization of the country’s air defense systems. About a third of Ukraine’s regions will be subject to the law. Under Ukrainian legislation, martial law also allows for a range of restrictions, including on the media and individual movement.
Critics of Poroshenko, who has been dogged by low approval ratings, said he is keen to implement martial law in an effort to delay presidential elections next March. In a phone call with NATO Secretary-General Jens Stoltenberg, the president said martial law would in no way impact the election.
The latest standoff comes after months of rising tensions between the two countries, which 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.
But this marks the first use of martial law since the beginning of the conflict, in which more than 10,300 people have died.
Ukraine’s Western allies sharply criticized Russia’s maritime maneuvers and called on Moscow to immediately release the captured Ukrainian soldiers.
Nikki Haley, the U.S. ambassador to the United Nations, called Russia’s actions “outlaw” and vowed sanctions punishing its annexation of Crimea would continue.
Speaking at an emergency meeting of the Security Council called by both Russia and Ukraine, Haley said the United States would welcome a normal relationship with Russia.
“But outlaw actions like this one continue to make that impossible,” she said. “The United States will maintain its Crimea-related sanctions against Russia. Indeed, further Russian escalation of this kind will only make matters worse.”
Haley noted she had spoken with President Trump and Secretary of State Mike Pompeo before coming to the Security Council.
Trump, however, offered a more tepid response, telling reporters, “We don’t like what's happening and hopefully it will get straightened out.”
Trump is expected to meet with Russian President Vladimir Putin on the sidelines of the G-20 summit in Argentina, which kicks off Friday.
Carol Morello in Washington contributed to this report.