Nairobi, Mogadishu - Somali pirates have hijacked a Japanese tanker and an Iranian cargo ship in the Gulf of Aden just two days after a Malaysian ship was seized, the International Maritime Bureau (IMB) said Thursday.

The BBC quoted IMB head Noel Choong as telling reporters in Kuala Lumpar that the ships were seized on Thursday morning.

There are believed to be around 20 crew onboard the Japanese ship while the number onboard the Iranian vessel is unknown.

Piracy off Somalia's lawless coast has surged in the last three months. Prior to the seizure of the Malaysian ship, the IMB said that eight vessels had been attacked in the region.

Two German citizens seized from their yacht in late June were recently released after a ransom of one million dollars was paid.

Somalia's weak transitional government, currently engaged in countering a bloody insurgency, is powerless to stop the pirates from seizing ships and cashing in.

"The situation in this region is grave," IMB Director Pottengal Mukundan said in a recent statement. "These pirates are not afraid to use significant firepower in attempts to bring vessels under their control."

Pirates, armed with rocket-propelled grenades and automatic weapons, use speedboats to pursue their targets.

The United Nations Security Council has approved incursions into Somali waters to fight piracy, and the IMB said it understood that coalition warships had helped prevent two attacks.

Somalia has been in a state of anarchy since the overthrow of dictator Mohamed Siad Barre in 1991. (dpa)