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.

Nearly 150,000 hectares (370,000 acres) of aquaculture farms and more than 20,000 floating fish cages are at risk from flooding and strong winds, according to state media.

A boy survived the sinking of a tourist boat in Vietnam which killed dozens of people by sheltering in an air pocket of the overturned vessel, it has emerged.