Somali pirates are demanding a total ransom of $8.2 million to free two Malaysian tankers and a Japanese-managed bulk carrier hijacked in the Gulf of Aden, a maritime official said.

Gunmen from Somalia have hijacked at least 30 vessels in the area so far this year, making the shipping lanes off the Horn of Africa nation the most dangerous in the world.

Meanwhile, Malaysia's national shipping line MISC has stopped its ships from entering the Gulf of Aden after two of its vessels were hijacked.

"MISC has with immediate effect put a halt on all its vessels ... entering the Gulf of Aden until additional security measures by MISC are in place to enhance the safety of its vessels and crew," MISC said in a statement.

MISC, part of state energy firm Petronas, has more than 100 ships. MISC said negotiations were continuing for the safe release of the crew of Bunga Melati Dua, which had 29 Malaysians and 10 Filipinos on board when it was seized on Aug 19.

Andrew Mwangura, head of the East African Seafarers' Assistance Programme, said the pirates wanted $4.7 million to release the Bunga Melati 5 and its sister ship, the Bunga Melati Dua.

He told Reuters the gangs were also demanding $3.5 million to free the MV Stella Maris, which was hijacked on July 20.

"We believe all three ships are near Eyl village, where the pirates have the strong support of locals," Mwangura said.

"Eyl was set up as a fishing base in 1973 and then later abandoned. It is a very remote area and the pirates feel safe. They know that no outsiders or spies can approach them there."    

Somali officials say gunmen are believed to be holding at least six vessels for ransom near Eyl. In total, the pirates are thought to be holding about 130 crew members hostage.

The Bunga Melati 5 was carrying 30,000 tonnes of petrochemicals to Singapore from Saudi Arabia when it was seized on Friday. It had 36 Malaysian and five Filipino crew on board.

MISC said it had been travelling "within the vicinity" of a Gulf of Aden security corridor that was set up last week, but that multinational forces had been unable to stop the hijacking.

The Bunga Melati Dua was laden with 32,000 tonnes of crude palm oil and was carrying 29 Malaysian and 10 Filipino sailors to Rotterdam from Indonesia when it was seized on August 19. Somali pirates also want a $1 million for a Nigerian tug boat, the MT Yenegoa Ocean, which was seized earlier this month.

Lawlessness onshore is spreading fast as Somalia collapses into the worst fighting for two nearly decades. The chaos is fuelling a wave of piracy that increasingly threatens vessels in the Gulf of Aden, one of the world's most important waterways.

Nearly 20,000 ships pass through the strategic channel each year, and the attacks are choking aid shipments to the country, worsening a crisis that aid workers say is the worst in Africa. - Reuters