The ocean of oil from U.S. shale drove crude prices back toward six-year lows Friday, and American energy companies say they are poised to unleash a further flood that would keep prices from returning to lofty levels for a long time.

The International Energy Agency reinforced the prospect of a prolonged slump in energy prices Friday, saying U.S. oil output was surprisingly strong in February and rapidly filling all available storage tanks. The Paris-based energy watchdog said this could lead to another sharp drop in crude...