While all sides agree to keep talking, little progress has been made in overcoming significant disagreements.
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.
Western officials have long said Iran must address the UN agency's suspicions about its work.
All sides agreed to continue talking for four more months to try reach a final agreement.
Six world powers are negotiating with Iran to scale back on enrichment capacity sharply.