Thai police are investigating the deaths of six foreign nationals whose bodies were found in a room at an upmarket hotel in Bangkok on Tuesday, including looking for a seventh person in connection with the incident.

All six, who were of Vietnamese descent, with two carrying U.S. passports, checked into Bangkok's Grand Hyatt Erawan hotel at two separate times after arriving on Saturday and Sunday, police official Thiti Saengsawang told reporters.

The group - three men and three women - checked into different rooms but their bodies were found in one room, which did not show any signs of struggle, he said.

"This was not self harm, but someone caused the deaths," said Thiti, adding that police were looking for a seventh person connected with the group.

"We are tracing every step since they got off the plane."

Police officers found the bodies after a call from the hotel staff at around 5.30 p.m. (1030 GMT) reporting that there had been deaths, the Thai police said in a statement.

Thai prime minister Srettha Thavisin, who visited the hotel late on Tuesday with senior police officials, ordered a swift investigation on the matter, the government said in a statement.

"The prime minister has ordered all agencies to urgently take action to avoid impact on tourism," it said.

The Grand Hyatt Erawan, which has over 350 rooms and is located in a popular tourist district in the Thai capital known for luxury shopping and restaurants, also did not immediately respond to calls or an email seeking comment.

Tourism serves as a key driver for Southeast Asia's second-largest economy, with the government expecting 35 million foreign arrivals this year after 28 million visited the country in 2023, spending 1.2 trillion baht ($33.71 billion).

The tourism sector was shaken last October by a shooting spree at a luxury shopping mall, close to the Hyatt, in which two foreigners were killed, prompting government measures to improve confidence, including ramping up security at popular locations.

To woo more visitors, the government has offered longer visa stay periods and waivers for several nationalities.