The explosion took place at around 11:00 pm (1600 GMT) on Wednesday in the Thakhek district of Khammuan province while the bus was en route to the Lao capital, Vientiane, from the southern town of Pakse.

Foreign ministry spokesman Sodom Petrasy said it was a planned attack.

"This is a normal situation in Laos because we have bandits who rob and steal goods from passengers," he told AFP on Friday.

Sodom was unable to say why "bandits" intent on theft would blow up a bus, nor was he was able to provide any details on the condition of the injured who were taken to the Khammuan provincial hospital.

Laos often uses the term "bandits" to describe ethnic minority rebels fighting the communist regime. It maintains that the country has never had an insurgency problem.

Two deadly attacks in February and April this year along the main north-south highway were also blamed on "bandits", although diplomats believe they were the work of Hmong rebels trying to draw attention to their cause.

This latest incident is likely to cast a further shadow over the impoverished country's tourism industry -- a vital source of hard cash -- particularly since it is the first attack in recent years in the south.

Laos experienced a series of 14 bombings between 2000 and 2001 in Vientiane in which four people were killed and more than 40 injured. No one claimed responsibility for those attacks.

Tens of thousands of Hmong, a tribal hill people, were recruited by the US to fight its secret war in Laos against North Vietnamese troops and Pathet Lao rebels during the Vietnam War.

But after the 1973 Paris Peace Accord which paved the way for US withdrawal from the region, the 10,000-strong Hmong fighters were abandoned.

The desperate rag-tag rebels, however, were never completely wiped out by the Lao army despite assistance from Vietnam, which officially maintained a military presence in Laos until 1989.

Diplomats have said that Vietnamese soldiers were actively fighting the Hmong in the northern mountains as recently as 2000.

The official Pathet Lao newspaper said the blast took place around 14 kilometres (nine miles) from Thakhek town.

It cited a passenger as saying the bomb went off while the 40-seater bus had stopped to change drivers. The force of the blast blew out the vehicles' windows.

The Communist Party mouthpiece said security forces in the area were searching for the culprits "in order to bring them to justice".

A western diplomat in Vientiane said he was unaware of any foreigners being caught up in the blast.

Two Swiss nationals and a Chinese citizen were among 13 people killed in an attack on a convoy of vehicles on Route 13 February 6 some 160 kilometres (100 miles) north of Vientiane near the town of Vang Vieng.

On April 20, 12 people were killed and 31 injured in another ambush on a bus along the same highway.

"Both incidents involved robberies carried out by armed bandits. Physical evidence shows that both of these incidents were robberies," Deputy Prime Minister Somsavat Lengsavad said at the time.