Visitors from several nations drowned before dawn on Thursday when their boat suddenly sank at one of the country's most popular tourist attractions.

Vietnamese media have identified the boat owners as Truong Hai Co, but the respected Thanh Nien daily reported the firm previously operated under another name until four months ago.

Thanh Nien, quoting an unidentified source, said the previous company owned a boat that sank on the bay in September 2009.

Three foreign tourists and a local guide died in that accident when their vessel overturned during heavy rain.

At the time, Thanh Nien reported that the new boat had breached several regulations, prompting authorities to pledge stricter enforcement of seaworthiness standards and registration rules.

Authorities have pledged a safety crackdown after the accident and said they will prosecute anyone responsible.