A boat has over turned in the picturesque Halong Bay in Vietnam killing twelve tourists, with Australian, British and American nationals among them.

The boat was exploring the UNESCO World Heritage Site in Quang Ninh province when it went down, with a Vietnamese guide also believed to be among the dead.

Two tourists from Sweden and one from Japan have also been confirmed dead, with the cause of the accident yet to be explained.

Fifteen people, including nine foreigners, were rescued from the water.

Survivors reported seeing a plank of the wooden live-aboard ship ripping away, followed by gushing water inundating the boat.

It went down around 05:00 near Titov Island, said Vu Van Thin, chief administrator of Quang Ninh province.

“Crew members tried to stop the water from coming in and alerted the tourists who were sleeping, but the water came in and the boat sank quickly,” he said.

“All of the 12 people who died were in the cabins.”

Halong Bay is renowned for its hundreds of tiny islands and freshwater swamp forests.

The area is one of the most popular tourist attractions in Vietnam.

More than 100 cruise boats are licensed to offer overnight service, and last year the area received 5.4 million visitors, nearly half of them foreigners, according to government websites.