The US has said threats by Iran to restrict Gulf shipping in the event of further sanctions shows international pressure is having an effect.
The State Department said sanctions on Tehran over its nuclear programme were starting to bite and that Iran was trying to create a distraction.
Iran has conducted 10 days of exercises near the strategically vital Strait of Hormuz, test-firing several missiles.
Its currency is at a record low, but it has denied sanctions are to blame.
The UN Security Council has already passed four rounds of sanctions against Iran for refusing to halt uranium enrichment.
Highly enriched uranium can be processed into nuclear weapons, but Iran denies Western charges that it is trying to develop them.
