Experts have been taking frequent measurements at several points on the Amur river where benzene levels have only once slightly exceeded authorized levels, the emergency situations ministry said in a statement.

Levels of another and less dangerous chemical, nitrobenzene, were five percent above authorized limits in one reading taken at Nizhne-Leninskoye on the right bank of the Amur, but also began to drop significantly late Sunday, the statement said.

The benzene slick, which last month floated down China's Songhua river following an explosion at a chemical factory in Jilin City, upstream of Harbin, on November 13, currently extends over 190 kilometers (118 miles) of Russian territory.

It was Monday about 100 kilometers (62 miles) west of Khabarovsk, a Russian city of around 600,000 people, where it was expected to arrive on Wednesday, the statement added.

While the emergency situations ministry has generally assessed the threat posed by the slick as low, other environmental groups have warned of long lasting consequences.

On Monday a member of Russia's lower house of parliament from Khabarovsk, Boris Reznik, took aim at Beijing, accusing the authorities of lax environmental controls.

"In the last several years 16 oil refineries have been built along the Songhua and other Chinese rivers as well as more than 70 cellulose and paper plants -- and not one of these has modern purification equipment," Reznik told the Echo Moscow radio station.

"All this filth -- chlorine-containing materials, pesticides, nitrobenzene -- flows into the Amur in huge concentrations," he said.

China has been embarrassed by the latest accident and the risk posed to its giant neighbour, amid a growing awareness of the environmental implications of the Chinese economy's rapid growth.

President Hu Jintao said earlier this month that China considered itself "highly responsible to the two countries and the two peoples."

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