The 500-tonne Marine 711 with about 40 Ghanaian and three South Korean sailors was boarded by unidentified pirates last Monday.

The pirates seized the three South Koreans and escaped on a separate speedboat, with their current whereabouts unknown.

The Marine 711, registered in Ghana, later arrived at a port at Ghana where the Ghanaian sailors disembarked, according to the South's Yonhap news agency.

South Korea's military said the pirates had been identified as Nigerians, Yonhap reported. Munmu the Great, a South Korean warship that had been involved in anti-piracy operations in the Gulf of Aden, had been sent to the nearby sea, it said.

"We are closely coordinating with local countries including Ghana, Nigeria, Togo and Benin as well as the US and the EU to locate our nationals and secure their release," the foreign ministry said.

The pirates have not yet demanded a ransom in return for the release of the abducted sailors.