At least six people were found dead at a hotel in Bangkok on Tuesday, a Thai police official said, adding that they were Vietnamese nationals.

The police official, speaking on condition of anonymity, dismissed earlier reports in Thai media that the six people had been killed in a shooting.

"There is no sign of a shooting," the official told Reuters without elaborating further, while some local media said the victims, including some with U.S. passports, died of cyanide poisoning, citing a police source.

Thai Prime Minister Srettha Thavisin said he has ordered an investigation into the incident.

(With input from Reuters)