PITTSBURGH (Reuters) - About 1,000 gallons of a biodegradable detergent spilled from a chemical plant into a southwestern Pennsylvania river, killing a small number of fish and prompting an investigation by state environmental authorities, officials said on Thursday.

The spill at the Henwil Corp. facility in Newell occurred when a tank overflowed into a cracked containment basin that leaked the cleaning compound into the Monongahela River on Wednesday evening, plant manager Tim Phelps said.

A total of 2,600 gallons of the soap-like substance spilled into the containment basin, and about 1,000 gallons or less escaped into the river, he said. The nontoxic chemical, which floated on the surface of the water, killed between 10 and 25 fish by clogging their gills, he said.

"Once that disperses away from the initial source, then there's no more harm to the waterways," Phelps said. "It's in no way toxic to humans."

The state Department of Environmental Protection dispatched an emergency response team and was conducting an investigation, said Katy Gresh, a spokeswoman for the agency.

The chemical was purchased by Henwil to use as an ingredient in its products, which are used in water treatment plants.

(Reporting by Daniel Lovering; Editing by Ellen Wulfhorst and Jerry Norton)