South Korean President Moon Jae-in ordered related government agencies and the military to "invest full efforts into the rescue operations, while putting the protection of lives first," the ministry said.
 
The fishing vessel that went out of contact was registered in Mongolia and owned by an ethnic Korean businessman living in South Africa. The vessel's captain, chief engineer and boatswain were South Korean nationals, the Foreign Ministry said.
 
Crews from South Korean fishing vessels and cargo ships have often been targets of Somali pirates.
 
Recent weeks have seen a resurgence of piracy off Somalia's coast after five years of inactivity. The piracy was once a serious threat to the global shipping industry but lessened in recent years after an international effort to patrol off the coast.
 
Some Somali fishermen, including former pirates, say foreign ships illegally fishing in local waters are forcing them to return to piracy to make money.
 
The 300-member Cheonghae Unit, led by a 4,400-ton destroyer, has been taking part in anti-piracy operations in the Horn of Africa since 2009.
 
In 2011, sailors and marines from the same unit raided a South Korean-operated cargo ship that had been hijacked by Somali pirates in the Arabian Sea, rescuing all 21 crew members and killing eight pirates and capturing five others. The five captured pirates were taken to South Korea and received long prison terms. (AP)