The slick has reached the junction of the Songhua and Amur rivers, known in Chinese as Heilong, which forms the border between the two countries, China's official Xinhua news agency said late Friday.

Harbin business owners and residents plan to take a class action lawsuit against the chemical plant to China's highest court, the South China Morning Post said on Saturday.

An explosion on Nov. 13 at Jilin Petrochemical Co., a unit of PetroChina, spilled 100 tonnes of cancer-causing benzene compounds into the Songhua River.

Government officials didn't reveal the 80-km (50-mile) slick's existence to the public until 10 days later, when it threatened water supplies in the city of Harbin.

Seventeen restaurant and public bathhouse owners and three Harbin residents would take their case to the Supreme People's Court next week, if provincial courts failed to hear it by Tuesday, the South China Morning Post said.

"I simply want to do justice to my fellow citizens in Harbin whose health has been under serious threats over the years by the contaminated river," Wang Baoqing, a restaurant owner seeking a symbolic compensation of 10,000 yuan ($1,238), told the paper.

"It appeared both provincial courts are adopting delay tactics while waiting for instructions from higher authorities, which may not come any time soon," the newspaper quoted Hu Fengbin, the lawyer leading the litigation, as saying.

"Despite (the fact) that there is no precedence in the country to provide compensation for large-scale environmental damages...we will give it a try," he said.