Responders to an oil-train derailment in West Virginia pulled more than 3,000 barrels of oil from tank cars, but said some oil had spilled into a river.

About a dozen of the 28 cars on a CSX line that derailed near Mt. Carbon, W. Va., last week were carrying crude oil. All told, that would be the equivalent of about 8,000 barrels of oil on board.

A unified command set up by rail company CSX, local, state and federal authorities said crews pulled about 3,600 barrels of oil directly from the tank cars as of Sunday.

"Both pumping and removal operations will continue around the clock pending safe work conditions and weather conditions, until both are complete," the unified command said in a statement. "Response teams will remove impacted soil and lay a new rail bed in order to rebuild the CSX railroad tracks."

The U.S. Environmental Protection Agency said a "small amount" of oil had spilled into the Kanawha River, though EPA officials said there was no impact on the safety of regional drinking water supplies.

None of the derailed cars fell into the river. Responders last week deployed containment boom as a precautionary measure and said additional measures would be used if necessary.

A string of derailments, including the deadly 2013 incident in Lac-Megantic, Quebec, sparked concerns over the use of rail cars designated DOT-111 to transport crude oil. The CSX cars were a newer model labeled CPC 1232.

The increase in U.S. crude oil production is more than the existing network of pipelines can handle, leaving the industry to rely more on rail as a transit option. A federal report found oil-by-rail volumes increased from 10,800 carloads in 2009 to 400,000 in 2013.

The report estimates about 10 "safety events" involving crude oil transportation by rail annually during the next 20 years.