A US official says world powers and Iran will start drafting a comprehensive nuclear agreement next month.
The official says the sides are making progress in talks.
The official was not authorised to speak for attribution and demanded anonymity.
The US and its partners hope to remove the threat of Iran developing nuclear weapons. Tehran says its programme is peaceful. It wants relief from the US and international sanctions.
The sides reached an interim deal last year providing Iran limited easing of sanctions for uranium and plutonium curbs.
The UN says Iran is honouring the agreement thus far. It expires in July and may be extended.
But the US official said the goal is to seal a final pact without the extension.
Negotiators meet again next week in Vienna.