The BBC's Duncan Hewitt: "Thousands have had to leave their homes"

Soldiers and civilians are building an emergency barrier to limit the flooding which has already swamped a tenth of the 20,000 wells in the Daqing field in Heilongjiang province.

Correspondents said the situation at the oilfield was grim, with breaches along the Nen River threatening to submerge the heart of China's oil industry.

An official with the Daqing oilfield Anti-Flood Office said emergency measures to drain water from a crucial reservoir were successful.

Race against time

Meanwhile troops are blowing up dykes and continuing to build others in a bid to keep the waters of the Songhua River from hitting Harbin, capital of Heilongjiang province.

The Songhua water level is at a record 120.44m and rising. It is expected to reach 120.60m by Friday, threatening Harbin with its worst floods in 50 years.

The city, at the edge of the river, has a population of about 9 million and is home to some of China's biggest industrial concerns.

Fear of epidemics

The flooding has created health and hygiene problems

The Chinese authorities are concerned about possible epidemics. The floods have killed more than 2,000 people, left tens of millions homeless and poisoned drinking water supplies.

Chinese officials said environmental pollution in the flooded areas would increase the possibility of epidemic diseases in the coming months.

But the Health Minister, Zhang Wenkang, said that although intestinal disease was on the increase, "major epidemic diseases have not yet appeared."

The financial cost

About 250,000 soldiers struggle to save China's largest oilfield

The Chinese authorities are also worried that a million people could be left in poverty following the devastating floods in central and north-eastern regions of the country.

Officials say the cost of the flood damage could be as high as $25bn.

The State Development and Planning Commission has called for efforts to stabilise prices and authorised provincial authorities to impose price caps on foodstuffs and building materials.