Chinese workers on Friday began to build a temporary diversion dam on the Fuyuan waterway of the Heilong River in a bid to prevent a toxic slick from reaching a nearby Russian city.

China and Russia agreed to build the temporary dam through friendly consultations after explosions in a chemical plant in Jilin Province last month caused water pollution of the Songhua River, which joins the Heilong River downstream.

The Heilong River flows down to Khabarovsk, a major city in Russia's Far East which gets water supplies from the river.

It is estimated that about 100 tons of carcinogens benzene and nitrobenzene fell into the Songhua River after the blasts.

The pollutant slick in the Songhua River has reached the Tongjiang section where the Songhua joins the Heilong River.

China's State Environmental Protection Administration (SEPA) said on Friday that nitrobenzene was found in Tongjiang at noon Thursday and by 6 a.m. Friday its was measured at a concentration of 0.0456 mg per litre.

Chinese and Russian experts are sampling water from the Heilong River, said the SEPA.

According to the sources, the temporary dam will be removed after the pollutant slick passes Khabarovsk along the Heilong River (or the Amur River in Russia).

China will bear the cost for the construction and demolition of the dam, the sources said.