Beijing Thursday called on Laos to bolster protection for Chinese citizens within its borders after six of its nationals were wounded by gunmen in the third attack of its kind this year.

The shooting took place north of the tourist hotspot Vang Vieng late Wednesday, when assailants opened fire on a Chinese passenger bus carrying 28 people from Kunming in southwestern China to the Laos capital Vientiane.

Six of the passengers were injured, two of them seriously, China's foreign ministry spokeswoman Hua Chunying told reporters.

"After the incident, the Chinese foreign ministry and embassy in Laos quickly launched representations to Laos, demanding Laos send military and police assistance to fully rescue the wounded... and to take concrete steps to strengthen the protection of Chinese citizens' safety," she said.

The foreign ministry batted down speculation that the victims were targeted on the basis of their nationality, despite two other similarly mysterious attacks this year that have killed three Chinese citizens and injured others in the mountainous region.

"When the criminals carried out their crimes it was not on the basis of nationality," Chua said.

The other attacks, in January and early March, prompted travel warnings from the US embassy in Vientiane.

The warning cited "the unpredictable nature of the violence and the lack of official information regarding possible motives or a Lao government response."

Laos is tightly run by its opaque Communist leaders, who bar a free press and have not offered a motive for the assaults.

The recent attacks have taken place in provinces historically home to outbreaks of insurgent violence waged by ethnic minorities against the country's repressive one-party state.

But Beijing's growing footprint in the poor nation has also stirred unease among locals in recent years.

China has invested heavily in Laos and capitalised on its bountiful water, forestry and mineral resources.

While this flood of foreign investment has fuelled impressive economic growth in the landlocked country over the past decade, the gains have not been evenly distributed and poverty remains widespread.

Normally isolated Laos will open its doors to host US President Barack Obama later this year, the culmination of its chairmanship of the ASEAN regional bloc.