A tour boat has sunk in Vietnam's scenic Ha Long bay, killing 11 tourists and a local guide, officials said.

Twenty-one foreign tourists were on the boat when it went down near Titov island, said Vu Van Bot, a police officer in Quang Ninh province. The total number of people on the boat was not immediately known.

Twelve bodies have been found, including those of tourists from the US, Australia and Britain, and have been sent to Bai Chay hospital for identification. The body of a Vietnamese tour guide was also recovered, said Ngo Van Hung, director of Ha Long bay's management board.

Nine foreign tourists and six Vietnamese were rescued, and efforts were under way to find others who may still be missing after the boat went down at about 5am on Thursday morning.

Giang Quoc Hung, the deputy director of the hospital, said three foreign tourists, including men from the US and France and a woman from Switzerland, were admitted to the hospital for treatment of minor injuries.

"They were in panic," Hung said. "They were given first-aid treatment and have returned to their hotels."

Online newspaper Dan Tri quoted Do Thong, the provincial vice governor, as saying the tourists on board were from 11 countries, including the US, Britain, France, Russia, Denmark and Sweden.

Police were contacting local weather forecasters to determine whether the sea was rough at the time of the incident.

Ha Long bay is a tourism hot spot about three hours east of the capital, Hanoi. Many visitors stay overnight on boats with sleeping cabins to cruise the picturesque bay, which is dotted with limestone formations.