The explosion occurred at 5am in Laos’ Khammuan province, 15 kilometres from the border checkpoint at Vietnam’s Quang Binh province.

The cause of this explosion remained a mystery as of press time. However, Pathedlao Daily, which belongs to state-run Lao News Agency, said in a Facebook post that the bus was loaded with illegally felled logs being smuggled out of Laos.

The news was initially broken by Facebook user Keovongvin Bounnhukhien, who posted several photographs of the accident. Vietnam’s Dan Tri International website also shared several photographs and posts from unknown Laos media sources showing smoke coming out of the burned out bus standing in the middle of the road.

Initial reports indicated that there were 12 casualties, with nine dead and three seriously injured. The victims were all passengers, and a source claimed that they worked for log smugglers. The nine deceased all hail from Vietnam’s Nghe An province.

“When I arrived at the scene, I could see the bus going up in flames with fragments shooting everywhere,” one eyewitness was quoted by Dan Tri as saying. Laos police say the explosion could have been caused by a bomb loaded on the bus or detonated by a suicide bomber on the bus, Dan Tri said.

Police have not ruled out other causes, it said.