Somali pirates have freed 23 Kaliningrad sailors and their Thai-flagged fishing trawler after four months in exchange for a "very significant" ransom.

The Russians were released Sunday from the port of Heradeere together with two Filipino and two Ghanaian sailors from the Thai Union 3 trawler, the European Union’s anti-piracy naval force said on its web site.

The ransom was "very significant," but the pirates called it a fine for illegal fishing in Somali waters, piracy expert Mikhail Voitenko said on his web site.

The crew was released as the result of joint efforts between the ship's owner and the Russian authorities, the Foreign Ministry said in a statement Monday, without elaborating.

"The crew's state [of health] doesn't raise serious concerns," the ministry said.

Pirates on two skiffs captured the Thai Union 3 as it was fishing for tuna about 200 nautical miles north of the Seychelles archipelago and 650 miles off the Somali coast.