A truck carrying ten barrels of TDI overturned in the county of Jiuzhaigou on the southern border with Sichuan Province early on Sunday morning, dumping all the containers into the adjacent Tangzhu River, which joins the trunk stream of Baishui River and flows northward into Wenxian County of Gansu, the environmental protection bureau of Wenxian said.

The Sichuan government informed Gansu of the accident in time and Wenxian immediately suspended the water supply from the river.

Eight barrels have been retrieved as of Thursday afternoon after joint operations by the two neighboring provinces, Wenxian government said.

Searches are underway to recover the two missing barrels.

TDI, a toluene-like chemical usually used as an ingredient of industrial paint, is harmful to people's respiration system, eyes and skin.

There has been no report of casualties due to the accident.

The county, which has a population of 243,000, had partially resumed its water supply by Thursday noon as monitoring results on five sections of the Baishui River did not show any sign of pollution, but the lower reaches of the river are under strict observation.

About 60,000 residents living along the river section in the county have turned to mountain springs for the past four days, but the water supply to the rest of the population was not affected.

The State Environmental Protection Administration has sent a team of officials to the county to deal with the accident.