MOSCOW — Since Russia fired shots at three Ukrainian Navy ships over the weekend, Western countries have issued statements saying they are “concerned” or even “deeply concerned,” but so far they have not — as Ukrainian officials had hoped — imposed sanctions.

The Ukrainian president, Petro O. Poroshenko, argued in an interview in the German newspaper Bild on Thursday for a more forceful response from the West, including for NATO warships to be sent into disputed waters in the Sea of Azov.

“Putin wants the old Russian Empire back,” Mr. Poroshenko cautioned, suggesting a new stage of Russian aggression. “Crimea, eastern Ukraine, he wants the whole country.”

“Germany also has to ask itself,” he added, “What will Putin do next if we do not stop him?”

Ukraine is not a member of NATO, whose focus has traditionally been on guaranteeing security against Russia. The alliance issued a statement condemning Russian forces for impounding Ukrainian vessels on Sunday — as the European Union did on Wednesday when it announced that the bloc would not impose sanctions — but NATO commanders have said entering the Sea of Azov is unlikely: It is not considered international waters.

The crisis flared less than a week before President Trump was to meet President Vladimir V. Putin of Russia, on the sidelines of a summit meeting in Argentina of leaders from the Group of 20 industrialized nations. On Thursday in Moscow, the Kremlin said the meeting was still scheduled, but Mr. Trump later wrote on Twitter that he had canceled it, “based on the fact that the ships and sailors have not been returned to Ukraine from Russia.”

The tensions flared as three Ukrainian vessels — two small gunboats and a tugboat — were trying to pass through the Kerch Strait, a narrow body of water that connects the Black Sea and the Sea of Azov. On Thursday, Ukraine’s domestic intelligence agency, the S.B.U., said Russia had fired on its ships from a fighter jet, a helicopter and from Coast Guard boats.

Since annexing the Crimean Peninsula from Ukraine in 2014, Russia fully controls the strait militarily, though Ukraine contends that under the terms of a 2003 treaty, the two countries share sovereignty over the Sea of Azov with equal rights to pass in and out.

Ukrainian officials say Russia is disregarding that treaty and asserting control over the Sea of Azov and whittling away at Ukrainian sovereignty.

Russian officials say that is not the case, adding and that the episode on Sunday turned violent because the Ukrainian ships had failed to follow protocols for entering the strait, such as notifying the Russian Coast Guard in advance.

The dispute continued to simmer on Thursday. Ukraine’s minister of infrastructure, Volodymyr Omelyan, wrote on Facebook that Russia was blocking 35 ships from entering or leaving the Kerch Strait in a de facto blockade of two Ukrainian ports in the Sea of Azov: Mariupol and Berdyansk.

Dmitri S. Peskov, Mr. Putin’s spokesman, said that the waterway was open for Ukrainian shipping but that the authorities might temporarily close it during rough seas.

The commander of the Ukrainian Navy suggested that countries retaliate against Russia by closing the Bosporus in Turkey to Russian military traffic, citing a 1930s treaty that might justify such a move. This, too, seemed unlikely: Russia uses the waterway to supply its military forces in Syria, and although Turkey and Russia back opposite sides in the Syrian civil war, Turkey has allowed this traffic.

Mr. Poroshenko also said on Thursday that he would close the land border on the isthmus of the Crimean Peninsula, between Ukraine and Russian-annexed territory, to all foreigners, preventing Russians from entering the area overland via Ukraine.

In the Bild interview, Mr. Poroshenko said that NATO could help enforce Ukraine’s rights to navigation. “Germany is one of our closest allies,” he said, “and we hope that states within NATO are now ready to relocate naval ships to the Sea of Azov in order to assist Ukraine and provide security.”

The German chancellor, Angela Merkel, responded on Thursday by saying that there was “no military solution” to the dispute.

The conflict at sea was “entirely the doing of the Russian president,” Ms. Merkel said in remarks broadcast by the BBC, adding, “issues like these can only be solved by sensible dialogue.”

Follow Andrew E. Kramer on Twitter: @AndrewKramerNYT.

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