* U.S., Britons, Swedes, French, Russians among dead
* Cause of sinking unclear, Halong Bay closed
(Adds detail, byline, changes dateline)
By Nguyen Huy Kham
HALONG BAY, Vietnam, Feb 17 (Reuters) - Twelve people including 10 foreigners drowned on Thursday when their tour boat sank in Vietnam's scenic Halong Bay in one of the country's worst accidents involving tourists.
Authorities were investigating what caused the "Truong Hai 06" to go down before dawn with 27 people on board while anchored on a calm night in the picturesque bay, one of Vietnam's biggest tourist attractions.
"Water poured into the boat at around five in the morning when the tourists were sleeping," said Le Nhu Thieu, deputy head of the office of the Quang Ninh provincial people's committee.
Police investigators and embassy staff were interviewing the 15 shocked survivors wrapped in blankets and taken ashore on boats.
Halong Bay, a UNESCO World Heritage Site dotted with more than 1,900 green-tinged islets, was closed off as bus loads of tourists arrived for overnight boat trips and cave tours that were abruptly cancelled.
The bodies of two Vietnamese people, two Americans, two Russians, two Swedes, a Briton, a French citizen, one Japanese and one Swiss were lined up, covered in white sheets, at a mortuary in the Bai Chay Hospital.
Thieu said the boat sent a distress signal for help as it went down and boats nearby came to the aid of those who were able to escape.
Photographs on the Quang Ninh provincial government website (www.quangninh.gov.vn) showed wetsuit-clad divers near what appeared to be the mast of the boat jutting out of the water in the foggy bay, its signature limestone crags in the background.
The sinking was the deadliest accident to hit Halong Bay, about 200 km (125 miles) northeast of Hanoi, an archipelago that draws tens of thousands of domestic and overseas visitors each year, most of whom take overnight boat tours.
The weather had been fine and the cause of the sinking remained unclear, Ngo Van Hung, director of the Halong Bay Management Department, told Reuters.
Vietnam received 4.6 million foreign tourists in the first 11 months of last year, up 36.5 percent from the same period in 2009 and exceeding the target of 4.5 million visitors for the whole of 2010, according to the country's tourism authority. (Additional reporting by Ngo Thi Ngoc Chau; Writing by John Ruwitch; Editing by Martin Petty and Robert Birsel)
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