A boat carrying tourists capsized during a sudden thunderstorm in Vietnam on Saturday afternoon during a sightseeing excursion, killing as many as 34 people, state media reported.

The Wonder Sea boat was carrying 48 passengers and five crew members — all of them Vietnamese — during the tour of Ha Long Bay, a popular destination for visitors, according to reports.

The number of reported rescues varied as the day went on, but the most recent figures published in local outlet VNExpress said 11 had been rescued.

The boat turned upside down because of strong winds, the newspaper said. A 14-year-old boy was among the survivors and was rescued four hours after being trapped in the overturned hull.

The newspaper said most of the passengers were tourists, including about 20 children, from the capital of Hanoi.

A tropical storm is also moving toward the area. A national weather forecast said Wipha is expected to hit Vietnam's northern region, including Ha Long Bay's coast next week.