HA LONG BAY, Vietnam: Rescuers were desperately searching for four people still missing on Sunday (Jul 20) after at least 35 were killed when a boat capsized in one of Vietnam's most popular tourist destinations.

The tourist boat ferrying families around Vietnam's famed Ha Long Bay was lashed by a storm on Saturday in one of the deadliest disasters at the UNESCO World Heritage site.

The vessel"Wonder Sea" vessel was carrying 46 passengers and three crew members when it capsized because of sudden heavy rain, according to a provincial police report seen by AFP.

Previous state media reports had said that 53 people were on board and 37 people had died, but the figures were later revised by police.

Tran Trong Hung, a resident in the Ha Long Bay area, told AFP:"The sky turned dark."

There were "hailstones as big as toes with torrential rain, thunderstorms and lightning", he said.

Most of those on board were families visiting from the capital Hanoi, with more than 20 children among the passengers, state media outlet VNExpress said.

35 bodies had been recovered and 10 people rescued by Sunday, police said, with four people still missing.

One of the rescued, a 10-year-old boy, told state media outlet VietnamNet:"I took a deep breath, swam through a gap, dived then swam up, I even shouted for help, then I was pulled up by a boat with soldiers on".

Prime Minister Pham Minh Chinh sent his condolences on Saturday to relatives of those killed and called on the defence and public security ministries to conduct urgent search and rescue.

Authorities would"investigate and clarify the cause of the incident and strictly handle violations", a government statement said.

CNA understands that no Singaporeans were on board the tourist boat.

Torrential rain also lashed northern Hanoi, Thai Nguyen and Bac Ninh provinces on Saturday.

Several trees were knocked down in the capital by strong winds.

The storm followed three days of intense heat, with the mercury hitting 37 degrees Celsius (99 degrees Fahrenheit) in some areas.

Mai Van Khiem, director of the National Center for Hydrometeorological Forecasting, was quoted in VNExpress as saying that the thunderstorms in northern Vietnam were not caused by the influence of Tropical Storm Wipha in the South China Sea.

Wipha entered the South China Sea on Sunday gaining strength, and is on course to make landfall in Vietnam early next week.

Ha Long Bay is one of Vietnam's most popular tourist destinations, with millions of people visiting its blue-green waters and rainforest-topped limestone islands each year.

Last year, 30 vessels sank at boat lock areas in coastal Quang Ninh province along Ha Long Bay after Typhoon Yagi brought strong wind and waves.