The benchmark U.S. oil price tumbled to a six-week low Friday, thwarting hopes for a sustained recovery after an influential energy watchdog said U.S. production growth is defying expectations and setting the stage for another bout of price weakness.

Investors and oil producers should brace for further declines in oil prices, the International Energy Agency said in a monthly report. Prices haven’t fallen far enough yet to cut supply, and some signs of rising demand are just temporary—bargain buyers using cheap oil to fill...