Eighteen retirees returning from holiday were killed on Tuesday after their tour bus plunged down a ravine in northern Thailand, police and a media report said, the latest disaster in a country with notoriously dangerous roads.

The victims were all former employees of state-run company CAT Telecom and were making their way to Bangkok from a province in the mountainous north.

They were returning to the capital after a holiday trip, the Bangkok Post website reported.

"18 people died and 20 were injured after a bus fell down into deep ravine because the driver was not familiar with the road," Police Colonel Ditsayadej Patcharapuwadol, commander of Uttaradit police station, told AFP.

"The bus was on its way to Bangkok -- coming down a steep road when it hit a barrier and fell into the 50-metre deep ravine," he said, adding that the driver was one of those killed in the accident in Muang district of Uttaradit province.

Photos of the crash site posted on social media showed the bus had come to a halt down a steep jungle ravine, its roof almost entirely sheared off.

Despite relatively good infrastructure, Thailand has the world's second most dangerous roads in terms of per capita deaths, according to data collected by the World Health Organization in a 2015 report.

The WHO estimates about 24,000 people die each year in traffic accidents on Thailand's roads.

Only Libya records a greater number of fatalities per capita.