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.