KIEV, Ukraine — The president of Ukraine is urging NATO to deploy warships to the Sea of Azov, a proposal that has been sharply criticized by Russia as a provocation that could worsen tensions between the two countries following a weekend confrontation in the waters off the Crimean Peninsula.

U.S. President Donald Trump said Thursday he was cancelling a planned meeting with Russian President Vladimir Putin at the G-20 summit in Argentina because Moscow has not released the Ukrainian vessels and sailors it seized.

In an interview published earlier Thursday with the German daily Bild, Ukrainian President Petro Poroshenko laid out his hope that NATO would “relocate naval ships to the Sea of Azov in order to assist Ukraine and provide security” against Putin’s expansionist ambitions.

The Russian coast guard fired on and seized three Ukrainian vessels and their crews on Sunday. Russia alleged the Ukrainian vessels had failed to obtain permission to pass from the Black Sea into the Sea of Azov through the Kerch Strait. Ukraine insisted its vessels were operating in line with international maritime rules in the strait, which separates Russia’s mainland and the Crimean Peninsula that it annexed from Ukraine in 2014.

While condemning the Russian action, NATO is not expected to send ships to the area, a deployment that could trigger a confrontation with Russia. A 2003 treaty between Russia and Ukraine stipulates that permission from both countries is required for warships from anywhere else to enter the internal sea.

NATO spokeswoman Oana Lungescu said the alliance already has a strong presence in the region, and that NATO ships routinely patrol and conduct exercises in the Black Sea, especially those from Bulgaria, Romania, and Turkey, which border the sea.

German Chancellor Angela Merkel said early Thursday that she plans to press Putin at the upcoming G-20 summit in Argentina to urge the release of the Ukrainian ships and crews and to de-escalate the situation.

“We can only resolve this in talks with one another because there is no military solution to all of these conflicts,” she said.

It was not clear whether Merkel knew of Poroshenko’s call for NATO’s deployment when she spoke.

Trump tweeted his decision to cancel a meeting with Putin this weekend. Trump, who was en route to Buenos Aires for the G-20 summit, said he would not be meeting Putin because “the ships and sailors have not been returned to Ukraine from Russia.”

The Kremlin said it has not been notified of a cancellation. Russian news agencies quoted Dmitry Peskov, Putin’s spokesman, as saying that the Kremlin only learned of the cancellation from Trump’s tweet.

Peskov said the cancellation means that Putin will have “a couple of more hours” for “useful meetings” with other leaders of the world’s 20 largest economies.

The Ukrainian government said Russia has blocked commercial traffic to and from Ukrainian ports on the Sea of Azov, which Russia quickly denied.

“Putin wants nothing less but to occupy the sea,” Poroshenko said. “The only language he understands is the unity of the Western world.”

Putin on Wednesday criticized the West for what he described as connivance with Ukraine’s “provocation.”

“The authorities in Kyiv are successfully selling anti-Russian sentiments as they have nothing else left to sell,” he said. “They can get away with whatever they do. If they want to eat babies for breakfast today, they will likely get served too.”

In response to Sunday’s events, Ukraine has imposed martial law in parts of the country. Putin accused Poroshenko of provoking the naval incident in a bid to impose martial law to shore up his sagging popularity and sideline competitors ahead of a presidential election in March.

Peskov said Poroshenko’s request for NATO warships is “clearly aimed at provoking further tensions,” adding that it was driven by “electoral and domestic policy motives.”

Poroshenko said that martial law wouldn’t restrict travel, cash withdrawals or currency purchases by Ukrainians, but Russians will face some unspecified constraints.

Ukraine’s Ministry of Infrastructure says Russia has blocked 35 merchant ships from leaving or entering the Sea of Azov since Wednesday.

The ministry said 18 ships have been stuck in the Black Sea, waiting to pass through the Kerch Strait into the Sea of Azov. Another 17 vessels were unable to leave the Ukrainian ports of Mariupol and Berdyansk on the Sea of Azov, it said.

Peskov insisted that Russia hasn’t imposed any restrictions on the ships’ passage, adding that a possible logjam could be linked to poor weather.

Ukraine’s intelligence agency, the SBU, said a Russian fighter jet and a helicopter fired rockets Sunday at the three Ukrainian vessels before they were captured — the first time an airstrike was reported.

“It’s a miracle the Ukrainian seamen have survived,” SBU deputy chief Oleh Frolov told reporters.

There has been growing hostility between Ukraine and Russia since Moscow’s annexation of the Crimean Peninsula from Ukraine in 2014. Russia has also supported separatists in Ukraine’s east with clandestine dispatches of troops and weapons. Fighting there has killed at least 10,000 people since 2014 but eased somewhat after a 2015 truce.

The naval incident marked the first overt clash between Russian and Ukrainian militaries since the 1991 collapse of the Soviet Union. It has fueled fears of a wider conflict and has drawn strong criticism of Russia from the U.S. and its allies.

Amid the tensions, the Russian military said it had deployed another batch of the long-range, S-400 air defence missile systems to Crimea.

Turkish President Recep Tayyip Erdogan said he discussed the possibility of a Turkish mediation to resolve tensions and had separate phone calls with Putin and Poroshenko on Thursday.

Asked about the Turkish offer, Peskov responded that “Moscow is grateful to all those willing to help de-escalate the tensions provoked by the Ukrainian side, but doesn’t see any need for mediation efforts.”

“Those who have such opportunities could help by exerting influence on the Ukrainian authorities,” Peskov said.

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Isachenkov reported from Moscow. Nataliya Vasilyeva in Moscow, David Rising in Berlin and Lorne Cook in Brussels contributed.

Yuras Karmanau And Vladimir Isachenkov, The Associated Press