Iran has followed through on a threat to accelerate its production of enriched uranium, the head of the U.N. atomic watchdog said on Monday, departing from his usual guarded language to say he was worried about increasing tension.

The assessment comes at a time of sharply increased U.S.-Iranian confrontation in recent weeks, a year after Washington abandoned an agreement between Iran and world powers to curb Tehran's nuclear program in exchange for the lifting of international financial sanctions.

Washington tightened sanctions from the start of May, ordering all countries and companies to halt all imports of Iranian oil or be banished from the global financial system. It has also dispatched extra troops to the region to counter what it describes as Iranian threats.