Two of the injured are said to be "close to death"

Metal drums containing what is thought to be mustard gas were found on a construction site in the city of Qiqihar in Heilongjiang province.

Most of the injured are construction workers and people who came into contact with the drums after they had been unwittingly opened.

China has protested to Japan about the incident, which cast a shadow over weekend events to mark the 25th anniversary of the signing of a peace treaty between the two countries to conclude WWII.

China's official media said 34 of the injured were in hospital, with two "close to death".

Chinese victims and their families were reported to have called for compensation from Japan.

A senior Japanese government official, Yasuo Fukuda, said Japanese experts were in Qiqihar to investigate.

"If it turns out that this incident came from the former Japanese army, then I think it is necessary that we handle the case accordingly," Mr Fukuda said.

The accident occurred last week when construction workers found five metal barrels. One was opened, causing an oil-like substance to spill into the soil, according to the official China Daily.

Workers later cut the barrels into pieces and sold them to a recycling facility. Polluted soil from the building site was then moved to other locations as part of the construction work, the paper said.

Japan's war-time record in China is a regular source of tension between the two governments, and popular Chinese anger.

Large quantities of chemical weapons were left behind by the retreating Japanese army, some of them buried or concealed.

Several cases have already been lodged by Chinese plaintiffs against the Japanese government, alleging damage or injury because of the weapons.