At least 34 people have died and eight are missing after a tourist boat ferrying families around Vietnam's famed Ha Long Bay capsized on Saturday, state media said.
The vessel was carrying 48 passengers and five crew members when it capsized in the UNESCO World Heritage site because of sudden heavy rain, the Dan Tri news site said.
Most of those on board were families visiting from the capital Hanoi, with more than 20 children among the passengers, a report on the VNExpress news site.
"Border guards rescued 11 people and recovered 18 bodies," it added.
Rescue efforts were set to continue into the night to find the dozens still missing.
Prime Minister Pham Minh Chinh sent his condolences to the families of the deceased and called on the defense 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 statement on the government website said.
Tran Trong Hung, a resident in the Ha Long Bay area told AFP: "The sky turned dark at around 2 pm."
There were "hailstones as big as toes with torrential rain, thunderstorm and lightning", he said.
Torrential rain also lashed northern Hanoi, Thai Nguyen and Bac Ninh provinces on Saturday.
In the capital Hanoi, several trees were knocked down by strong winds.
The storm followed three days of intense heat, with the mercury hitting 37 degrees Celsius 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.
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.
Earlier this month, a ferry sank off the popular Indonesian resort island of Bali, killing at least 18 people.