Two Swedes were among those killed when a Vietnamese tour boat sank Thursday in Vietnam's Halong Bay, the Swedish government said.

"Two Swedish young women are dead," Anders Jörle, a spokesman for Sweden's foreign ministry, told AFP by telephone from Stockholm.

He said they were in their early 20s.

The Swedes were among 12 people, most of them foreign tourists, who died when their boat suddenly sank while they slept in the bay, a UNESCO World Heritage site renowned for its striking limestone towers.

Vu Van Thin, a senior official with the Quang Ninh province People's Committee, the local government, said the accident happened when "part of the boat suddenly broke", but bad weather was not to blame.