Croatian Prime Minister Andrej Plenkovic on Tuesday condemned Russia's capture of Ukrainian ships in the Kerch Strait, calling it an "unacceptable violation of international law".

Tensions between Russia and Ukraine have escalated after Russia had captured three Ukrainian naval vessels and their crews a week ago as they tried to pass through the Kerch Strait between the Black Sea and the Sea of Azov.

Russia has ignored international calls to release 24 Ukrainian sailors, including three who have been wounded in the incident, claiming that they had crossed the Russian border illegally.

“We are concerned about the situation in Ukraine… That is unacceptable, and contrary to international law,” Plenkovic said in his opening remarks at the second day of the Central European Initiative (CEI) summit in Zagreb.

“We support all international initiatives to ease tensions, while fully supporting Ukraine’s territorial integrity and sovereignty,” he added.

Security and migration were among the topics discussed at the summit, which wraps up Croatia’s year-long rotating presidency of the 18-country intergovernmental forum.

On irregular migrations, Plenkovic said that the problem could be tackled effectively only if approached comprehensively.

“We are not a country that has decided to put up physical barriers at the border. Instead, we have deployed 6,500 border police to guard the border, as we prepare to join the EU’s Schengen area,” Plenkovic said.

Although Croatia had joined the European Union in 2013, it is still not a member of neither its passport-free travel Schengen area or the euro zone.

Five prime ministers of CEI countries arrived in Zagreb on Monday, with Hungarian Prime Minister Viktor Orban and Romanian Prime Minister Viorica Dancila have left before the summit ended.

The CEI was established in Budapest in 1989 with a view to assisting countries in the region in joining the European Union. Of the 18 CEI member states, 10 have joined the EU, five are on track to becoming members of the bloc, while three are members of the Eastern Partnership.

The CEI members are: Albania, Austria, Belarus, Bosnia and Herzegovina, Bulgaria, Croatia, Czech Republic, Hungary, Italy, Macedonia, Moldova, Montenegro, Poland, Romania, Serbia, Slovakia, Slovenia, and Ukraine.

The next country to hold presidency of CEI is Italy.

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