The thunderstorm came so suddenly that it overturned the boat within seconds, said two survivors of the shipwreck which killed at least 35 people in Vietnam's top tourist spot of Halong Bay and left another four missing.

A Vietnamese man who survived the capsizing of a tour boat in Ha Long Bay described his escape from the overturned vessel. At least 38 people were killed in the accident and a search is continuing for five who are still missing, according to Vietnamese state media.