(Adds U.S. Navy comments, paragraphs 5-7)

BOSASSO, Somalia, Dec 4 (Reuters) - U.S. and German navy ships have cornered Somali pirates who seized a Japanese-owned chemical tanker more than a month ago and are demanding a ransom, an official said on Tuesday.

The Panama-registered Golden Nori was carrying benzene from Singapore to Israel when it was hijacked on Oct. 28, just off Somalia, one of the world's most dangerous shipping lanes.

At the time, U.S. Navy said coalition naval forces had pursued the pirates, opening fire and destroying speedboats the hijacked vessel had in tow.

"One German and two American warships have been after Golden Nori for 42 days after it was hijacked. The pirates arrived in Bosasso around 10 p.m.," Saeed Mohamed Rage, Puntland region's fishing and marine minister, told a news conference in the northern port town of Bosasso.

In Bahrain, a U.S. Navy spokeswoman said: "We are monitoring the situation after following the Golden Nori since the hijacking."

"We continue to encourage the pirates to free the crew and the vessel," said Commander Lydia Robertson.

The U.S. Navy, which has had a long presence in the region, is concentrating its anti-piracy efforts along the central coast of Somalia after several ships were hijacked there in recent months, she said.

Rage said there were 21 crew members on board.

"We are negotiating with their captors. We told them to surrender to Puntland authority," Rage said, adding that the pirates were demanding an undisclosed ransom.

"We are in good communications with the Americans and we are still engaged in dialogue with the hijackers," he said.

Deprived of central government since 1991, Somalia has gained a reputation for its piracy, despite calls for international action to patrol its waters. (Reporting by Abdiqani Hassan, and Firouz Sedarat in Dubai)

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