The bodies of many tsunami victims killed in Thailand are now so badly decomposed it is hard to tell if they are locals or tourists.
The job of identifying bodies has become much harder as they decompose
The Thai government says it is currently unsure of the origin of more than 2,000 of the bodies recovered.
Local temples have been turned into makeshift morgues as Thai and overseas forensic experts try to identify some of the thousands of bodies.
The effort mounted to identify the corpses is among the biggest ever.
The Thai interior ministry said examinations of 1,973 bodies had proved unreliable and further tests were being carried out.
Many tests had been conducted on samples that had already begun to decompose, and were being repeated on any bodies found more than five days after the disaster, Interior Minister Bhokin Bhalakula said.
Some 9,000 people are feared dead, and 5,305 bodies have so far been recovered. Of these, 1,792 are now thought to be Thai, compared with 2,578 on Saturday, and 1,329 foreign, compared with the previous figure of 2,516, the ministry said.
The origin of the other 2,184 is now uncertain - a figure ten times higher than it was on Saturday .
Notices about the missing are still plastered on walls
The revised figures came as international forensic teams and Thai specialists stepped up their efforts.
"We are working to international standards and the process has speeded up considerably," a Swiss expert said.
Computer chips are being implanted in bodies to ensure there are no mix-ups.
Forensic experts have taken dental X-rays and where possible, fingerprints. They have also taken DNA samples - often through extracting two teeth - which can be cross-matched against personal belongings or DNA from living relatives for formal identification.
The Thai coastline is where many of the foreigners killed or missing were staying.
The forensic teams were initially overwhelmed by the scale of the task. They had insufficient storage spaces for corpses, which were rapidly decomposing in the sweltering heat.
Bodies of local people are being burned instead of buried
There had also been problems with communication between the foreign experts and Thai forensic staff.
"At the start we had two different systems on the go. The Thais felt we were checking on their work and were offended," said Andrea Quattrini, liaison officer for the Swiss Body Identification Team.
Now the teams are able to process up to 40 bodies a day from one facility at the resort town of Krabi alone.
The initial work may be finished in as little as two weeks, but the formal identification may take months.
Information has to be cross-referenced among 30 countries which have reported their nationals as missing.
Foreigners' bodies are to be flown back to their native countries for burial.