Thailand's junta chief apologized on Thursday for suggesting tourists in bikinis could be more vulnerable to attack, comments that caused an international outcry following the brutal murder of two British vacationers.

Prayut Chan-O-Cha, who is also prime minister, on Wednesday drew condemnation after questioning the safety of female tourists in the kingdom in off-the-cuff comments to government officials.

"They think our country is beautiful and is safe so they can do whatever they want, they can wear bikinis and walk everywhere," Prayut said.

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

The remarks came just two days after the battered bodies of British tourists David Miller, 24, and Hannah Witheridge, 23, were found on the southern resort island of Koh Tao.

In a rare public moment of contrition from the tough-talking army chief, Prayut said he did not mean to cause distress.

"I'm sorry that it hurt people," Prayut told reporters in Bangkok.

"I didn't intend to insult or criticize anyone. I just warned that sometimes people have to be careful ... today Thailand is safe except there are some bad guys - like anywhere in the world."

But he stopped short of retracting the comments about tourists wearing bikinis, instead seeking understanding for words uttered while "under pressure".

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

The incendiary words from Prayut, who seized power from the elected government in May, prompted the British embassy in Bangkok to ask for a "clarification" raising its "concerns" over his remarks.

John Sifton, Asia Advocacy Director at Human Rights Watch, branded the comments "demeaning".

Speaking before Prayut issued an apology, he said the remark "unfortunately implies that women victims of violent crimes are somehow responsible for the abuse they suffered because of what they were wearing.

"This sort of statement demeans women - and if that was not his intent, then he should issue a statement clarifying matters."

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

On Thursday, police continued to search for clues on the small, normally laid-back diving island of Koh Tao as post-mortem examinations of the victims' bodies in Bangkok failed to find 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 told AFP.

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.

Thailand's key high season for tourists is just two months away and the kingdom is desperate to reassure visitors after a year that saw tourism receipts slump because of political turmoil.