Russia seized Ukrainian ships and sailors last week and has been accused by Kiev of restricting the movement of vessels through the narrow Kerch Strait, which connects the Sea of Azov to the larger Black Sea.

Federica Mogherini said that the EU was "suffering an impact" and she would raise the matter with the Russian and Ukrainian foreign ministers at meetings in Milan.

"We are extremely concerned about developments in the Azov Sea, not only because of the attempts on Ukraine but also because the activities in the Azov Sea slow down enormously the vessels that are also carrying European Union member states' flags," Mogherini said as she arrived for talks with NATO foreign ministers.

"So we are suffering an impact on our own economies beyond the security threats that we clearly see today."

Late last month Russia opened fire on and seized three Ukranian navy vessels as they tried to pass through the Kerch Strait.

Russian forces detained 24 Ukrainian crew members, three of whom were wounded.

The incident was the most dangerous in years between the ex-Soviet neighbours, which have been locked in a confrontation since 2014 when Russia annexed Crimea and supported an insurgency in eastern Ukraine.

Despite international calls for their release, the Ukrainians were sentenced to two months detention and are now held in Moscow.

Ukrainian Foreign Minister Pavlo Klimkin used talks on Tuesday with his NATO counterparts to press the alliance to come up with a "comprehensive answer" to the Russian security threat.

Wary of exacerbating tensions or entangling themselves in a complex conflict, European leaders have rebuffed Ukrainian calls for more support against Moscow.