Soldiers and emergency workers Friday struggled to retrieve the thousands of chemical- filled barrels that have entered a major river in northeast China, amid concern some of the barrels may have sunk to the bottom of the river. By noon Friday, 2,978 barrels - some empty, some filled with chemicals - had been recovered from the Songhua River that runs through northeast China's Jilin and Heilongjiang provinces. Workers stationed at eight points on the river have complained trash, weeds and tree branches floating in the river have hampered their work. At one of those points, where boats are chained together to form a block, engineers used four cranes to remove the floating debris before soldiers and experts on boats used long poles and steel nets to retrieve the barrels. Heilongjiang Province has dispatched soldiers and workers to establish six more points on the upper reaches of the Songhua River to retrieve the barrels. The soldiers and workers will use assault boats and cranes in their task, said Dai Jun, an army officer at the scene. But officials are worried because helicopters being used to track the barrels have lost sight of some of them. Experts fear some of the barrels may have sunk into the river -- making their retrieval a more arduous task. A video posted on Chinese web portal Sina.com Thursday showed a barrel catch fire and explode on the river's surface. Still, a water test conducted early Thursday morning showed the river water was not contaminated. More work crews and equipment have been mobilized to help in the retrieval work on the river. Some 3,000 full barrels -- filled with 170 kilograms of chemical liquid each -- and 4,000 empty ones were swept into the waterway after floods hit warehouses of two chemical factories in Jilin City, Jilin Province, Wednesday morning. 