Seoul - Pirates have seized a Japanese freighter off the coast of Somalia and are holding its 23-member crew captive, the South Korean Foreign Ministry said Sunday.

A group of gunmen boarded the 20,000-ton Chemstar Venus on Saturday when it was sailing in waters 155 kilometres east of the port of Aden, the ministry said.

Five Koreans were among the crew, the rest were Filipinos.

Somalia, which has no effective government, has become notorious for piracy in recent years, prompting the deployment of international naval vessels.

Two suspected Somali pirates were shot and killed by the crew of a British navy frigate in the Gulf of Aden on Tuesday after the sailors tried to board a dhow that had attempted earlier to seize a Danish vessel.

Pirates have hijacked 30 ships this year in the Gulf of Aden in the Arabian Sea between Yemen and Somalia, one of the world's main shipping routes, demanding high sums of ransom from the vessels' owners.