Officials said the sandbag dam, one of two built hastily to prevent the possible flow of benzene-contaminated water from the Amur river into Khabarovsk, a city of 600,000 in Russia's Far East region, would be completed during the night before the poison reached the city.

Two Russian military Mi-26 helicopters were flying non-stop to ferry sandbags to the site of the dam on the Kazakevich channel, one of many small branches of the Amur river west of Khabarovsk, to complete the last few meters of the 300-meter (985-foot) barrage across the waterway.

A spokesman for the Russian emergency situations ministry quoted by Interfax said there were around 3,000 Chinese laborers using 200 heavy construction trucks working on the project in cooperation with the Russians.

Local officials said the dam would be completed before the first traces of the spill could reach Khabarovsk.

Analyses of the water in the Amur river west of the city were being taken constantly -- 63 tests were conducted at various points on the river since Monday, officials said -- and no traces of toxic benzene had yet been detected.

However the spill, caused by an explosion at a chemicals factory in China on November 13, was expected to arrive in Khabarovsk soon after midnight Wednesday (1400 GMT Tuesday), though the concentrations of benzene and the consequences for the city's residents were difficult to predict.

The Amur river, which runs along the Russia-Chinese border before veering exclusively into Russian territory, splits into numerous smaller branches above Khabarovsk and effects on the city will depend on which of these channels it enters and how strongly it is concentrated, experts say.

In addition to the makeshift dam nearing completion Tuesday, a second barrage hastily erected in another branch of the Amur, the Penzenskaya channel, was completed last week, Russian officials said.

Benzene is a known carcinogen used as an industrial solvent and as a component of gasoline. More than 100 tons of the chemical were dumped into the Songhua river -- a Chinese tributary of the Amur -- following the blast at the plant outside Harbin.

While most Russian experts forecast negligible effects from the spill on the population of Khabarovsk, nearly all have described it as a major ecological disaster that will have consequences on the environments along the Songhua and Amur rivers for years to come.

In the weeks that it has floated down the river, the spill has stretched in length to more than 190 kilometers (120 miles) and it was expected to take the better part of a week for it to pass through Khabarovsk, experts said.

More than a month of advance warning however allowed local residents and authorities in Russia to preposition huge stocks of potable water in the event that the city has to shut down public water treatment facilities, as was the case in Harbin last month.

Local officials reiterated Tuesday that they wanted to see China pay compensation to Russia for the spill.

"We are spending money constantly," Viktor Ishayev, governor of the Khabarovsk region, said late Monday. He said public money spent in recent weeks on rush construction of the Russian dam alone had exceeded 60 million rubles (two million dollars).

"People are working on this every day. So of course all of this money will be added up in order to demand compensation," he said.

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