NAIROBI, Kenya — Somali pirates who seized a Japanese tanker six weeks ago have threatened to kill the 22 crew members unless a $1 million ransom is paid, a maritime official said Monday.

The Golden Nori is carrying crew from the Philippines, South Korea and Myanmar.

“Somali pirates holding the Japanese ship are demanding US$1 million,” said Andrew Mwangura, head of the Kenya-based East Africa Seafarers’ Assistance Program. “They are also saying we are going to kill the crew if our demands are not met.”

Deaths are rare during such hijackings.

The chemical tanker was anchored in Somali waters and carrying up to 40,000 tons of highly explosive benzene. The U.S. Navy in late October came to the aid of the vessel, with the guided missile destroyer USS Porter at one point opening fire to destroy pirate skiffs tied to it. The pirates, though, remained aboard the tanker.

The U.S. military has recently intervened several times to help ships hijacked by Somali pirates.

Somali pirates are trained fighters, in some cases linked to powerful Somali clans, outfitted with sophisticated arms and equipment. They have seized merchant ships, ships carrying aid, and once even a cruise ship.

Somalia has not had an effective central government since dictator Mohamed Siad Barre was overthrown in 1991. Now the weak transitional government and its Ethiopian allies are battling an Iraq-style Islamic insurgency. The chaos means that pirate ships can cruise the ragged coastline with relative impunity.