David Miller and Hannah Witheridge
David Miller and Hannah Witheridge were found dead in ThailandFacebook

Thailand's prime minister has said the country is not always as safe as home for foreign visitors, after two British tourists were murdered on the island of Koh Tao.

Thailand is a popular tourist destination and the sector accounts for around 10% of the country's gross domestic product, although the number of visitors has slumped since the government was ousted in a coup in May.

Thai police have said the bodies of David Miller and Hannah Witheridge were found naked with serious wounds on Monday, on a Koh Tao beach.

"We have ordered police and soldiers to hunt down the suspects," said General Prayuth Chan-ocha, who led the military coup in May.

"What (the media) need to do is explain to foreigners that it is not always safe everywhere. They think it's safe like where they are from, everywhere and at any time, but our country still has problems," Prayuth told reporters, as cited by Reuters news agency.

Thailand has been under martial law since the May coup, causing visitor numbers to fall as many travel insurers do not cover countries under military rule.

Around 15.7 million foreigners visited Thailand in the first eight months of the year, down 10% on the same period in 2013.

A spokeswoman for the Tourism Authority of Thailand had earlier said the incident was "out of character" for the island, adding that "Koh Tao and other beach and island destinations remains safe for tourists to travel to."

"We have limited details on the circumstances at this time and are waiting for further information following the initial stages of the police investigation. Our thoughts and condolences are with the families and friends affected by the news," the spokeswoman said in a statement.