Family members of Hannah Witheridge comfort each other at the headquarters of the Royal Thai Police in Bangkok on Thursday. Police said the bodies of David Miller and Witheridge were found naked with grisly wounds early on Monday on a beach on Koh Tao, a southern island famous for coral reefs and diving.

AFP/Bangkok

Britain on Thursday raised concerns after Thailand's junta leader suggested tourists in bikinis could be more vulnerable to attack, as DNA tests failed to yield a breakthrough in the hunt for the killers of two British holidaymakers.

The battered bodies of David Miller, 24 and Hannah Witheridge, 23, were found on the southern resort island of Koh Tao on Monday.

Distraught relatives of Witheridge broke down in tears and hugged each other ahead of a police briefing in Bangkok on Thursday, as the investigation continued to draw a blank.

Thailand's military ruler Prayut Chan-O-Cha on Wednesday drew condemnation after questioning the safety of female tourists in the kingdom.

"They think our country is beautiful and is safe so they can do whatever they want, they can wear bikinis and walk everywhere," Prayut - who is also prime minister - told government officials.

But "can they be safe in bikinis... unless they are not beautiful?"

The British embassy in Bangkok said it had contacted the Thai foreign ministry raising its "concerns" over his remarks and to ask "for clarification", according to an embassy statement.

Prayut's off-the-cuff comments echoed others made to reporters on Tuesday questioning the behaviour of the murdered Britons as well as the perpetrators.

Thailand's image as a tourist haven, already tarnished by months of political protests that ended in May's army coup, has been further damaged by the murder of the Britons.

On Thursday, police continued to hunt for clues on the small, normally laid-back diving of island of Koh Tao as post-mortem examinations of the victims' bodies in Bangkok did not find any DNA links to 12 people they have questioned so far.

Those include two of Miller's British friends who were asked to stay in Bangkok pending the forensic results - and several Myanmar migrant workers.

The British men were now "free to return home," regional police commander Panya Maman said.

Experts tested traces of semen and a hair found at the crime scene but could not find a match.

With no arrests and an apparent lack of new leads, it is unclear where the police investigation can turn after nearly four days scouring sparsely populated Koh Tao.

Related Story