The bodies of six people have been found at a luxury hotel in Bangkok and their deaths may have been caused by poisoning, according to police and officials in Thailand.

Bangkok police chief Lt Gen Thiti Sangsawang identified the dead as two Vietnamese Americans and four Vietnamese nationals, and said there were three males and three females.

Investigators said the bodies were foaming at the mouth.

The victims had booked several rooms at the Grand Hyatt Erawan Hotel under seven names, and some were staying on a different floor from the room where they were found dead, Lt Gen Thiti said.

Police are still looking for the seventh person included in the booking, he said at a news conference at the hotel. He said there were no signs of a struggle.

The residents of the room where the bodies were found were supposed to have checked out earlier on Tuesday and their luggage had already been packed, he said. The bodies were discovered by a maid who went to the room after they failed to check out and found it locked from the inside, the police chief said.

There was food that had been ordered earlier from room service that was left uneaten, but drinks had been consumed, he said. He would not confirm a cause of death, but said the deaths appeared to have occurred about 24 hours before police arrived on the scene on Tuesday evening after being called by hotel staff.

Prime Minister Srettha Thavisin went to the hotel in the evening, and later told journalists that the incident was neither a robbery nor a random assault, and that it should not impact Thailand’s lucrative tourism industry.

Pending the results of autopsies, “our hypothesis is that they ingested something that killed them”, Mr Srettha said.

US State Department spokesman Matthew Miller said officials are aware of the reports of the deaths of two US citizens in Bangkok.

“We offer our sincere condolences to the families on their loss. We are closely monitoring the situation and stand ready to provide consular assistance to those families,” he said.