copyright Getty Images Image caption Ukrainian forces have been mobilised in the port of Mariupol on the Sea of Azov
Ukrainian President Petro Poroshenko has urged Nato to send ships to the Sea of Azov following a naval confrontation with Russia off Crimea.
He told Germany's Bild newspaper he hoped the ships could be relocated "to assist Ukraine and provide security".
On Sunday, Russia opened fire on three Ukrainian ships and seized their crews in the Kerch Strait between the Black Sea and the Sea of Azov.
Nato has expressed "full support" for Ukraine, which is not a member state.
Amid worsening relations on Wednesday, Russian President Vladimir Putin accused Mr Poroshenko of creating the naval "provocation" to boost his ratings ahead of 2019 elections.
President Poroshenko has implemented martial law across Ukraine's border regions for 30 days in response to the crisis.What did Poroshenko say?
In the interview with Bild, Mr Poroshenko said Vladimir Putin wanted "nothing less than to occupy the [Azov] sea".
"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 said.Image copyright AFP Image caption President Poroshenko inspected tank crews during a visit to Chernihiv, northern Ukraine
"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?"
On Monday, Nato chief Jens Stoltenberg called on Russia to free the Ukrainian ships and sailors and said Moscow should realise the "consequences of its actions".
He said the bloc would continue to provide "political and practical support" to Ukraine, which is a Nato partner country.
Nato did not immediately respond to Mr Poroshenko's latest statement.What happened off Crimea?
At least three Ukrainian sailors were wounded when Russian FSB border guards opened fire on two Ukrainian gunboats and a tug off Crimea.
The peninsula was seized from Ukraine in 2014 and annexed by Russia shortly afterwards.
The naval boats had been sailing from Odessa to Mariupol, a Ukrainian port on the Azov sea, when they were confronted by the FSB vessels.
Both countries agreed to share the sea in a 2003 treaty, but Russia's decision to open a bridge across the Kerch Strait this year has exacerbated tensions.
Ukraine says Russia is deliberately blockading Mariupol and another port, Berdyansk, preventing ships from getting through the Kerch Strait.
The 24 captured Ukrainian sailors have now been given two months in pre-trial detention by a court in Crimea.What did Putin say?
"It is undoubtedly a provocation," the Russian president said, adding that it had been organised by Ukraine's authorities "and, I think, the incumbent president in the run-up to the Ukrainian presidential election in March 2019".
Mr Poroshenko has low popularity ratings. Recent polls suggest that only about 10% of the electorate plans to vote for him next year, with and nearly 50% saying they would not vote for him under any circumstances, the Kyiv Post newspaper reported.
Media playback is unsupported on your deviceMedia captionWhy tensions between Russia and Ukraine are so high
Mr Putin added that the Ukrainian president's decision to impose martial law after a mere "border incident" had not even taken place at the height of the conflict with pro-Russian separatists in eastern Ukraine in 2014.
He insisted that Russia's military response was appropriate as the Ukrainians had "trespassed" into Russia's territorial waters.
Ukrainian officials published a map on Wednesday, placing all three Ukrainian boats just outside Crimea's territorial waters at the time they were seized.What other reaction has there been?
Western governments have backed the Ukrainian government.
On Wednesday the EU strongly condemned Russia's actions but failed to agree on new sanctions against Russia.
After three days of debate, the EU issued a statement expressing "utmost concern about the dangerous increase of tensions" and dismay at the "unacceptable" use of force by Russia.
It called on Russia to release the ships and their crews and respect Ukraine's sovereignty. Poland had wanted new sanctions on Moscow while Germany and France stressed the need to ease tensions.
US President Donald Trump has also said he may cancel a planned meeting with Mr Putin on the sidelines of the G20 summit in Buenos Aires later this week.