"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. 

"We need to conduct an autopsy to see if they had ingested anything," he said, dispelling rumors the deaths were connected to a theft and had involved a shooting.  

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

The case was reported at 4:30 p.m. 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.

The chief of Bangkok's Metropolitan Police Bureau, Thiti Sangsawang, said the guests had failed to check out of the hotel that afternoon. 

A preliminary examination found no evidence of injuries related to a fight or theft but suggested all six had ingested a toxic substance, Thiti said.

"We need to find out the motives," he said, adding that the deaths were not the result of suicide, but of a "killing."

Earlier a police officer speaking on condition of anonymity denied initial reports that the six had been killed in a shooting, saying the investigation was currently focused on a "link with a toxic substance." 

Some of the six foreigners appeared to be on their first trip to the country, while others appeared to be return visitors, he added.

In a statement, the police said they were still investigating the scene and cause of death.

In Washington, State Department spokesman Matthew Miller expressed "our sincere condolences to families on their loss" and said the agency was "closely monitoring the situation" and was "ready to provide assistance to those families." 

The area where the crime took place is popular with tourists and home to several upscale shopping malls and the Erawan shrine, a visitor draw.

Last October, a 14-year-old boy opened fire at the Siam Paragon mall in the area, killing three people.

The shooting came just days before the anniversary of a massacre at a nursery in northern Thailand that left 36 people dead and amid government efforts to bolster tourist numbers.

Some 28 million people visited Thailand in 2023, up from 11 million the year before, but still well down from the 40 million who came in 2019, the last year before the pandemic.

Travel restrictions during the COVID-19 pandemic hammered the kingdom's vital tourism sector and arrivals have not bounced back as quickly as officials hoped.

Officials are hoping to hit 35 million visitors in 2024, with a target of $55 billion in revenue.