Bruce's lengthy statement continued, "Prior to the United States' successful military operation, Iran was amassing a growing stockpile of highly enriched uranium for which there was no credible peaceful purpose, and it was the only state producing highly enriched uranium, up to 60%, that does not have nuclear weapons."

The decision means that international inspectors will not be able to oversee sites. Experts have warned that Tehran could revive plans to build a bomb.

Oil prices rose 3% on Wednesday as Iran suspended cooperation with the U.N. nuclear watchdog and the U.S. and Vietnam reached a trade deal, but a surprise build in U.S. crude supplies limited price gains somewhat.