Six people have been found dead in a luxury Bangkok hotel after a suspected deliberate poisoning.

The three men and three women - four Vietnamese nationals and two Vietnamese Americans - were in a room locked from the inside.

There were no signs of a struggle and it's not being treated as a robbery.

Police chief Thiti Sangsawang said room service food was untouched but drinks had been consumed - with "unidentified substances" in the bottom of all six cups.

Four of the bodies were in the living room and two in the bedroom, he added.

They were found by a maid after one failed to check out.

The case is being treated as a likely murder rather than suicide, said Mr Sangsawang.

He said all the victims arrived at the Grand Hyatt Erwan Hotel on 13 and 14 July and were staying in five different rooms.

It's believed they could be related as their surnames are similar.

"We are tracing every step since they got off the plane," Mr Sangsawang told reporters.

However, one person appears to be unaccounted for as the rooms were initially booked for seven people.

Police are trying to check if that person entered Thailand.

Thailand's Prime Minister Srettha Thavisin visited the scene and told reporters the victims were found at around 4.30pm local time on Tuesday.

He said it was believed they had been dead for about 24 hours.

"From initial dead body inspection, the cause of dead could be from something they consumed," said Mr Thavisin.

The waiter who delivered the food to the room at around 1pm on Monday is being questioned.

Police said they were waiting for an embassy representative to go through the group's luggage, which had all been packed and taken to the room where they were found.

The US State Department said it was aware of reports that two of its citizens had died.