“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.
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 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.
Putin also turns to Germany
In a phone conversation with Chancellor Angela Merkel, Putin also said he hoped the German leader could intervene to rein in Kiev.
Putin “expressed a serious concern over Kiev's decision to put its armed forces on alert and to introduce martial law,” the Kremlin said in a statement following the call.
He also said he hoped “Berlin could influence the Ukrainian authorities to dissuade them from further reckless acts,” it added.
German Foreign Minister Heiko Maas on Monday proposed that France and Germany mediate a major crisis between Moscow and Kiev after Russian vessels boarded and captured three Ukrainian ships.
Speaking on a visit to Madrid, Maas said that France and Germany would “strive together, and if needed as mediators, to avoid the conflict leading to a serious crisis” as country representatives met with their Russian and Ukrainian counterparts in Berlin.