Bangkok (AFP) - Thailand's military ruler on Wednesday questioned whether tourists in bikinis are safe in the kingdom, in comments following the murder of two Britons whose battered bodies were found on a Thai island.

David Miller, 24, and Hannah Witheridge, 23, were found dead on the southern island of Koh Tao on Monday, sparking a hunt for their killers. Post-mortem examinations were carried out on Wednesday after the bodies were brought to Bangkok.

But authorities have yet to make an arrest despite questioning several suspects including two British men -- who are believed to have travelled with Miller -- and a number of Myanmar migrant workers.

"There are always problems with tourist safety. They think our country is beautiful and is safe so they can do whatever they want, they can wear bikinis and walk everywhere," Prime Minister Prayut Chan-O-Cha, who is also army chief, told government officials.

But "can they be safe in bikinis... unless they are not beautiful?" he said, addressing the issue of tourist safety in a speech broadcast live on television.

Prayut, who seized power from an elected government in a May 22 coup, is well known for making off-the-cuff remarks.

His statement appeared to echo others made to reporters on Tuesday in which he questioned the behaviour of the murder victims as well as the perpetrators.

Thailand is desperate to avoid further damage to its lucrative tourism industry, which has been battered in recent months after a prolonged political crisis ended in the coup.

In a message to Britain's foreign secretary published later Wednesday, deputy prime minister Tanasak Patimapragorn expressed his "profound regret and sadness" at the deaths.

"(The government) attaches highest priority to protecting safety of foreign nationals residing and visiting Thailand," he said, adding that Prayut had instructed authorities to conduct a "transparent and thorough" investigation into the case.

- DNA breakthrough? -

Forensic investigators are awaiting the results of DNA tests on a blonde hair found in Witheridge's hand and on traces of semen, according to the findings of a post-mortem examination carried out on Wednesday.

"The results are expected within 24 hours so everything will become clear tomorrow (Thursday)," forensic police chief Pornchai Sutheerakhun told reporters after the autopsy.

"The female victim suffered cuts to her head... while the male was beaten on the head... but water found in his lungs suggests he may have died from drowning," he said, adding cuts on Miller's hands showed signs of a struggle.

The wounds were inflicted by "a sharp, hard object... and (they were) hit hard with a rock", he added.

A bloodied garden hoe was also found near the crime scene.

Earlier Wednesday a provincial police commander said an unidentified Asian man captured by security cameras on the night of the killings was being treated as their "prime suspect". But he later toned down his remarks.

"Every group (person) is still under suspicion" Kiattipong Khawsamang, Surat Thani provincial police commander, told AFP, including two British travellers stopped at the capital's main airport late Tuesday.

The two men in their 20s have been asked to remain in Bangkok until the case is resolved.

In a statement the British embassy in Bangkok said it was "aware of reports that Thai police had spoken to British nationals in connection with the case".

"We stand ready to provide consular assistance if required," it said.

Police have pinned hopes on DNA results yielding a breakthrough in the three-day investigation.

But conflicting details over the focus of the police inquiry, released by different figures in a force which rarely centralises information, have created a confused picture.

Prayut on Tuesday urged investigators to conclude the cases swiftly and raised concern at the impact on the country's image.

With the start of the tourist high season just two months away, the junta had vowed to restore the nation's reputation as the "Land of Smiles", embarking on a clean-up of resorts after a series of complaints about scams, assaults and even police extortion.