BANGKOK, July 16 (Reuters) - 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.