BEIJING (AFP) - A suspected bomb attack in Laos killed two Chinese citizens and injured one more, Beijing's official news agency, Xinhua reported Monday.

The world's second-largest economy has been pouring money into the sleepy South-east Asian nation, a fellow Communist state, over the last several years, and became its largest investor in 2014.

Xinhua identified one of the victims as an employee of a Chinese mining company operating in the mountainous centre of the country.

The victims were on board a vehicle when the incident happened on Sunday, it cited the Chinese embassy in the Laotian capital, Vientiane, as saying.

The blast, which it repeatedly described as an "attack", occurred in Xaysomboun province, it said.

Unexploded ordnance is an enduring problem in Laos, a key battleground during the Vietnam War, when US warplanes dropped more than two million tonnes of munitions in some 580,000 bombing missions aimed at cutting North Vietnamese supply lines through the country.

An estimated 30 per cent of the devices failed to detonate and 50,000 people have been killed by the explosives since the end of the war.

The Chinese embassy said in a statement that two of its staff had visited the injured survivor, a civilian surnamed Zhou, in hospital.

It urged Vientiane to protect the safety of Chinese people and their property.

A special envoy of Chinese president Xi Jinping is scheduled to go to Laos this week, Xinhua reported separately, in the wake of a visit by US Secretary of State John Kerry, who landed in the country on Sunday as the US courts its former wartime foe.

President Barack Obama is due to travel to the rural, landlocked nation later this year for a summit of South-east Asian countries, marking the first visit by a US president.