Soldiers and emergency workers Friday were struggling 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 7:30 p.m. Thursday, nearly 1,500 barrels - some empty, some filled with chemicals - had been recovered from the Songhua River. The salvage work continued overnight but authorities have not updated the figure for the number of barrels retrieved.

Workers stationed at eight points on the Songhua River have complained trash, weeds and twigs floating in the river have hampered their work.

At one of those points, where boats are chained together to form a block, engineers use four cranes to remove the floating debris before soldiers and professionals on boats use long poles and steel nets to retrieve the barrels.

But officials are worried because some of the barrels being tracked by helicopters have disappeared.

Experts fear some of the barrels may have sunk into the river -- making their retrieval a more arduous task.

Still, a water test conducted early Thursday morning showed the river water was not contaminated.

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.