Thai authorities have released a final report into the mysterious deaths of five tourists and a tour guide in the northern city of Chiang Mai earlier this year.

But despite a five-month investigation, the report failed to identify the exact cause of the deaths.

The report says that four people who died at the Downtown Inn in Chiang Mai in January and February may have been exposed to the same toxic chemical or gas but they cannot be sure.

"As difficult as it may be to accept, ultimately, the precise causes for the deaths and illnesses cannot be definitively identified or confirmed," the report said, following the investigation by various Thai ministries with the help of the World Health Organisation.

The report listed rodenticide, used in pest control, and chemicals such as those found in pesticides as possible causes.

Authorities said they could not exclude the possibility that an elderly British couple, found dead at the same hotel as two of the other victims, had also been poisoned, although investigators could not establish a direct link.

It was originally speculated that food poisoning may have caused the deaths.

But Thailand's Department of Disease Control said three women, from New Zealand, the US and Thailand, "likely died of exposure to pesticides".

Another person, who died at a different hotel, had similar symptoms but again no definite link has been found, despite an investigation involving experts in Thailand, the US and Geneva.

The strange series of deaths first attracted attention when New Zealander Sarah Carter died in February and her two friends became ill.

It then came to light that four other foreigners and a Thai guide had died within a four-week period.

Ms Carter's father says the report sidesteps the issue of accountability.

Richard Carter says he hopes Thai authorities carry through on their promise to increase control on the use of pesticides in hotels and markets.

New rules are slated for chemical use that will be implemented in hotels and market areas.

"The risk will hopefully be a bit less now that they've started to take it seriously," he said.

He has also welcomed plans to more closely monitor and investigate any future tourist illnesses.

Chiang Mai health authorities say the Downtown Inn remains open, but said visitor numbers had fallen dramatically.