Ukraine's president has urged Nato to deploy naval ships to the Sea of Azov amid a stand-off with Russia.
"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," Petro Poroshenko said in an interview with German newspaper Bild.
Mr Poroshenko's request comes after the Russian coastguard on Sunday fired on and seized three Ukrainian vessels trying to pass through the Kerch Strait, which separates Russia's mainland and the Crimean peninsula it annexed from Ukraine in 2014.
Ukraine insisted its vessels in the Black Sea were operating in line with international maritime rules, while Russia said they had failed to get permission to pass.
While Nato condemned the Russian action, the allies will be unlikely to heed Mr Poroshenko's request, which could trigger a confrontation with Moscow.
A Nato spokesperson said on Thursday it already has a strong presence in the Black Sea and that alliance ships routinely patrol and exercise in the region.
Noting its vessels have spent 120 days there this year compared to 80 in 2017, Oana Lungescu said: "There is already a lot of Nato in the Black Sea, and we will continue to assess our presence in the region."
Kremlin spokesman Dmitry Peskov said Mr Poroshenko's request is "clearly aimed at provoking further tensions" and driven by the Ukrainian leader's "electoral and domestic policy motives".
A 2003 treaty between Russia and Ukraine stipulates that agreement from each of the two countries is required for warships from any other country to enter the internal sea.
Mr Poroshenko has responded to the incident by ordering martial law in much of the country, a move that went into effect with parliamentary approval.
Mr Putin accused his Ukrainian counterpart of provoking the naval incident in a bid to use martial law to shore up his sagging popularity and sideline competitors ahead of a March election.
On Wednesday, Ukraine's top diplomat, Andrij Melnyk, told Germany and other western states to "put Putin in his place" by extending sanctions against Moscow, banning energy imports and putting a planned gas pipeline between Russia and Europe on hold.
“Germany must take a clear line ... and put Putin in his place,” Mr Melnyk told German radio on Wednesday. “Everything is at stake.”
The ambassador even raised the possibility of sending German marines to the region.
“In military terms, what can you do? Sending German marines to the coast of Crimea ... could help stop an escalation. If you are there, Russians have fewer possibilities to act so brutally,” he said.
German chancellor Angela Merkel said she planned to raise the issue with Mr Putin at this weekend's G20 summit in Argentina.
"We can only resolve this in talks with one another because there is no military solution to all of these conflicts," she told a Ukrainian business summit in Berlin on Thursday.
Meanwhile, Recep Tayyip Erdogan said he has discussed the possibility of a Turkish mediation to resolve tensions between Moscow and Kiev.
The Turkish president made the comments hours after he held separate phone conversations with Mr Putin and Mr Poroshenko.
"Can we assume a mediator role? We discussed this subject with both sides," he said.
Ukraine has insisted its vessels were operating in line with international maritime rules, while Russia said they had failed to get permission to pass.
Kiev has released what it said was the exact location where its ships were fired on by Russia, saying they were in international waters west of the Kerch Strait.
Russia, meanwhile, insisted the Ukrainian vessels were in its territorial waters and refused to communicate with the Russian coast guard or accept a Russian pilot to guide them through the narrow strait.
"What were the border guards supposed to do?" Mr Putin said on Wednesday. "They fulfilled their duty to protect the territorial integrity of the Russian Federation. If they had done something differently, they should have been put on trial for that."
The showdown comes amid the long-simmering conflict between the two countries, in which Russia annexed Ukraine's Crimean peninsula in 2014 and supported separatists in Ukraine's east with clandestine dispatches of troops and weapons.
The fighting there has killed at least 10,000 people since 2014 but eased somewhat after a 2015 truce.
Additional reporting by agencies
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