The warning was issued at a board meeting of the UN’s International Atomic Energy Agency in Vienna.
The US last week penalised a number of Iranian and foreign companies for violating sanctions against Tehran.
All sides agreed to continue talking for four more months to try reach a final agreement.
US Secretary of State John Kerry met Irain’s Mohammad Javad Zarif on Tehran’s nuclear programme.
Six world powers are negotiating with Iran to scale back on enrichment capacity sharply.
Iran, the US, Britain, France, Germany, Russia and China included the 20 July deadline to reach a pact.