Ukraine's President Petro Poroshenko has urged the western allies to deploy naval vessels to the Sea of Azov after Russian forces fired on and seized three Ukrainian boats in waters off Crimea on Sunday.

Western leaders have denounced the Russian act, demanded Moscow return the boats and crews and restated their rejection of Russia's 2014 annexation of Crimea, but they are wary of escalating the latest incident.

The crisis will, however, take centre stage next week, when the foreign ministers of the 29-member Atlantic Alliance hold their own regular meeting -- and meet Ukraine's Foreign Minister Pavlo Klimkin.

"The NATO-Ukraine Commission met this week to discuss developments near the Sea of Azov, and NATO foreign ministers will meet with Ukrainian Foreign Minister Klimkin next week," spokeswoman Oana Lungescu said.

NATO Secretary General Jens Stoltenberg, who will host next week's meeting on Monday and Tuesday at NATO's Brussels headquarters, spoke to Poroshenko on Monday.

The alliance has not directly addressed Poroshenko's request for naval assistance in the already tense waters off Crimea, but the spokeswoman stressed that NATO is already present in force 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.

"Today, NATO ships routinely patrol and exercise in the Black Sea. In 2018, NATO ships spent 120 days in the Black Sea, compared to 80 in 2017."

Three NATO members -- Romania, which hosts a multinational brigade, Bulgaria and Turkey -- have Black Sea coasts and their own forces, and alliance aircraft carry out regular air patrols in the region, she said.

"So there is already a lot of NATO in the Black Sea, and we will continue to assess our presence in the region. NATO and its Allies continue to give strong political and practical support to Ukraine," Lungescu said.