Ukrainian President Petro Poroshenko asked NATO members including Germany Thursday to send naval vessels to the Sea of Azov to back his country in the standoff with Russia.
"Germany is one of our closest allies, 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," he told Germany's Bild daily.
President Vladimir Putin insisted Wednesday that Russian forces were right to seize three Ukrainian ships last weekend.
But Poroshenko charged that Putin "wants nothing less than to occupy the sea. The only language he understands is the unity of the Western world.
"We cannot accept this aggressive policy of Russia. First it was Crimea, then eastern Ukraine, now he wants the Sea of Azov.
"Germany, too, has to ask itself: What will Putin do next if we do not stop him?" he added on the day Ukraine's Prime Minister Volodymyr Groysman was to visit Berlin.
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Russia fired on and then seized three Ukrainian ships on Sunday, accusing them of illegally entering its waters in the Sea of Azov, in a dramatic spike in tensions that raises fears of a wider escalation.
Kiev accused Russia, which annexed Crimea from Ukraine in 2014, of launching "a new phase of aggression".
NATO chief Jens Stoltenberg on Monday demanded Russia free the Ukrainian ships and sailors, warning Moscow that "its actions have consequences".
Poroshenko also told the German newspaper that German "Chancellor (Angela) Merkel is a great friend of Ukraine.
"In 2015, she already saved our country through her negotiations in Minsk, and we hope she will once again support us so strongly, together with our other allies.
"Putin wants to bring back the old Russian Empire. Crimea, Donbas, he wants the whole country," he added.
"As a Russian emperor, as he sees himself, his empire cannot function without Ukraine, he sees us as a colony.