Moscow (AFP) - European leaders rebuffed calls from Ukraine for greater support against Russia on Thursday, after Kiev urged NATO to send ships into waters disputed with Moscow.

German Chancellor Angela Merkel asked Kiev to be "sensible" following the request from President Petro Poroshenko, just hours after the European Union failed to agree to threaten new sanctions against Moscow.

Kiev has called on Western allies to back expressions of support with concrete action after Russian forces on Sunday seized three Ukrainian ships off the coast of Crimea.

The incident was the most dangerous in years between the ex-Soviet neighbours -- who are locked in conflict over Russian-backed separatist regions -- and has raised fears of a wider escalation.

In an interview with Germany's Bild newspaper published Thursday, Poroshenko asked NATO members including Berlin to send naval vessels to the Sea of Azov to back his country.

"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.

But, while blaming Russia for tensions, Merkel showed no signs of being ready to back military support.

"We ask the Ukrainian side too to be sensible because we know that we can only solve things through being reasonable and through dialogue because there is no military solution to these disputes," she said.

- Putin accuses Kiev of 'provocation' -

Ukraine has also urged Western governments to impose more sanctions on Russia over the incident, but there too it has seen little support.

In a statement on Wednesday, the EU strongly condemned Russia's actions but, after three days of debate among senior officials, could not agree on a tougher response.

Wednesday's statement reiterated demands that Russia release the seized boats and their crews, and restated Europe's "full support for the independence, sovereignty and territorial integrity of Ukraine".

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It stopped short of threatening sanctions, saying only that the EU "is determined to act appropriately, in close coordination with its international partners".

NATO spokeswoman Oana Lungescu said the issue would be raised at next week's regular meeting of alliance foreign ministers in Brussels, where Ukrainian Foreign Minister Pavlo Klimkin is also expected.

She stressed that NATO is already present in the wider region.

"Since Russia's illegal annexation of Crimea in 2014, NATO has substantially increased its presence in the Black Sea," she said.

Moscow and Kiev have traded angry accusations since Russian navy vessels fired on, boarded and captured the three Ukrainian ships.

After warning of the threat of "full-scale war", Poroshenko on Wednesday signed an act imposing martial law for 30 days in regions bordering Russia, the Black Sea and the Sea of Azov.

Russian President Vladimir Putin insisted Moscow's actions were "lawful" because the Ukrainian ships had crossed illegally into Russian territory.

He said the incident was a provocation orchestrated by Poroshenko to shore up public support ahead of a tough re-election battle in a presidential vote set for March.

- Russia denies blockading sea -

The Kremlin on Thursday also denied claims from Kiev that it was blocking access for Ukrainian ships to the Kerch Strait, the only crossing point between the Black Sea and the Sea of Azov.

"I don't know of any restrictions at the moment. On the contrary, as far as we know the Kerch Strait is operating normally," Putin's spokesman Dmitry Peskov said during his daily briefing.

Ukrainian Infrastructure Minister Volodymyr Omelyan said on Wednesday that 18 Ukrainian vessels were awaiting permission to pass from the Black Sea to the Sea of Azov.

Another nine vessels were waiting in port on the Sea of Azov to pass in the other direction.

"Ukrainian ports on the Sea of Azov... are effectively under blockade by the Russian Federation," Omelyan wrote on Facebook.

US President Donald Trump late on Thursday called off a planned meeting with Putin at the G20 in Argentina this week over Moscow's detention of the Ukrainian sailors.

"Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting in Argentina with President Vladimir Putin," he wrote on Twitter.

The Ukrainian vessels -- a tug and two gunboats -- were trying to pass through the Kerch Strait from the Black Sea to the Sea of Azov, but were refused access and chased into international waters by Russian vessels.

Kiev has demanded the return of its ships and the release of 24 sailors taken prisoner during the confrontation.

The sailors have been put before a court in Simferopol, the main city in Russian-annexed Crimea, and ordered to be held in pre-trial detention for two months.

Two of their lawyers said Thursday that the sailors had been transferred to Moscow for holding in the city's notorious Lefortovo prison.