Thai authorities have begun investigating the deaths of six foreign nationals in a luxury hotel room in central Bangkok.

Thailand's Prime Minister Srettha Thavisin said police suspect they were poisoned.

"There were no signs of a struggle," Srettha told a press conference at the Grand Hyatt Erawan hotel in the upscale Pathum Wan district, where the incident took place.

He added that autopsies will be carried out on the deceased to rule out cause of death.

The prime minister said all six of the deceased were Vietnamese, two of whom had dual US nationality.

The case was reported after cleaning staff discovered the bodies when they arrived to make up the room on the fifth floor, police said.

Srettha arrived on the scene shortly after. He said it was believed a seventh Vietnamese person could have been involved in the incident.