At least 35 people died after the vessel overturned in a heavy thunderstorm during a sightseeing tour.

A man who survived the capsizing of a tour boat in Ha Long Bay has told how he escaped by swimming out through a window underwater.

Dang Anh Tuan, 36, was on holiday with 11 university friends, when the Wonder Sea overturned on a sightseeing tour during a sudden thunderstorm.

Only three of his group survived, while the nine others, including a man who was travelling with his wife and three-year-old son, were among those killed.

The Vietnamese government initially said 38 people had died, but later revised the toll down to 35.

Four other people remain missing from the boat, authorities said, after cutting the estimate of those aboard from 53 to 49.

Mr Tuan said a storm quickly covered the sky, bringing high winds and pouring rain, as the boat embarked on a three-hour trip.

He said the passengers asked for the boat to turn back to shore, but the crew reassured them they were almost at their destination and kept moving forward.

"The boat capsized in less than a minute. It rained for about 15 minutes, and then the boat started to shake vigorously, tables and chairs were jostled around and seconds later the boat overturned," he said.

"Water gushed in and I lost all orientation. I tried to breathe. But more water came in.

"I took a deep breath, got rid of my life vest and dove down. I saw a streak of light and followed it to swim out, escaping the boat, and then I climbed on the overturned boat to look for help."