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 today 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.
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.
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His statement appeared to echo others made to reporters yesterday in which he questioned the behaviour of the murder victims as well as the perpetrators.
In a message to Britain's foreign secretary published later today, 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.
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 today.
"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.