Thai police have said cyanide poisoning caused the deaths of six foreign nationals whose bodies were found in a room at a luxury hotel in central Bangkok on Tuesday.

The deceased were three men and three women of Vietnamese descent in their 30s to 50s. Two of them carried US passports.

The police said in a news conference on Wednesday that cyanide was found in six coffee cups and in the blood samples collected from one of the deceased men.

Investigators said the bodies had no obvious wounds and there were no signs of struggle.

The police said they believe the six were likely poisoned to death but they have not found any evidence indicating the involvement of a third party.

The incident took place in a popular tourist district of the Thai capital filled with shops and offices. Prime Minister Srettha Thavisin has urged a swift probe into the case.