Bangkok: A British man has returned to the murder scene of two of his fellow backpackers on a scenic Thai resort island and is being questioned by police.

The man identified by Thai media as Christopher Alan Ware travelled to Bangkok after the murder of his British travelling companion David Miller and another British tourist on the island, Hannah Witheridge, both aged 24.

Mr Miller and Ms Witheridge were hacked to death with a garden hoe in a rocky alcove on Koh Tao in the Gulf of Thailand in the early hours of Monday.

Mr Ware, who had been sharing a room with Mr Miller, was initially cleared of suspicion in the killings and has not been formally named as a suspect.

Police are awaiting the results of forensic tests being conducted on the bodies in Bangkok.

Mr Ware's apparently voluntary return to the island came as Thailand's military junta leader leader and prime minister Prayuth Chan-ocha warned tourists about travel safety in a country popular with Australian tourists, and appeared to call into question the behaviour of the victims.

Ms Witheridge and Mr Miller had attended a beach party before they were killed.

Telling police to find the killer or killers swiftly, General Prayuth said: "We have to look into the behaviour of the other party too, because this kind of incident should not happen to anybody and it has affected our image".

"Local people must tell tourists when the safe times are to be outside, we have to help them understand," he said.

"In their countries (foreigners) can travel wherever they want, so they thought it is safe, but in our country, there are still problems…there are different types of people, so they have to be careful."

Despite the imposition of martial law across the country following a coup in May, all-night parties on beaches like Koh Tao and neighbouring Koh Phangan have seen rapes, robberies, drug arrests and shakedowns, according to consular staff of foreign embassies in Bangkok.

Many of the crimes are not reported to police.

Larry Cunningham, Australia's recently retired honorary consul based in Phuket, has said that Australian parents would be shocked to know how Thai criminals target young Australians and other foreigners at parties like those on Koh Phangan, where each full moon up to 30,000 ravers cram into one kilometre of beach.

"They are some of the worst criminals in Thailand…rapists, murderers and thieves and some are corrupt police," he said.

"These are dangerous, dangerous places."

General Prayuth made the comments despite a 10 per cent fall in tourist arrivals in the country where tourism accounts for 10 percent of the economy.

Martial law has kept many visitors away as they are unable to buy travel insurance for destinations where martial law is in place.

General Prayuth said there are no plans to lift the law.

Since ousting Thailand's democratically-elected government following months of political unrest, the junta has moved to crackdown on prostitution, sleaze, drug taking and police extortion in resort areas.

On Phuket, an island where up to 20,000 Australians holiday each month, armed soldiers have supervised the removal of illegal beach structures, including restaurants, and targeted mafia-type gangs operating rip-off taxies and tourist scams.

Regional Surat Thani governor Chatpong Chatphum said the Koh Tao murders had dealt a serious blow to tourism.

He said more security cameras would be installed on beaches.

Hours after the bodies of Mr Miller and Ms Witheridge were found suspicion swung to six migrant workers from Myanmar.

But they were released after questioning.

Police also want to speak with an Asian-looking man seen in blurred security camera footage walking back and forth near the crime.

Police say Ms Witheridge appears to have been dragged alive about 15 metres after she was attacked by the killer or killers, who may have attempted to raped her.

She was then bludgeoned to death with the hoe that was found near the bodies.

Photographs of the bodies have been circulating on the internet.

Mr Miller, a civil engineering student at Leeds University, had met Ms Witheridge while both were staying at the island's Ocean View Bungalows.

More than 70 police officers have been sent to the island 410 kilometres south of Bangkok, a popular diving destination that is also called Turtle Island.