Iran and six world powers are working to finalize the terms of a likely extension of negotiations over Teheran's nuclear program beyond a July 20 deadline. An announcement may come as early as Friday, Western diplomats said.

Officials from both sides have said it appears the talks will not yield a breakthrough by the self-imposed target date after two weeks of efforts failed to bridge gaps in positions over a deal intended to end a dispute that has persisted for decades.

Several diplomats close to the negotiations in Vienna suggested they expected talks to resume in September.

Western nations fear Iran's nuclear program may be aimed at developing a nuclear weapons capability. Teheran says it is for peaceful purposes.

"We hope to be gone before Sunday and wrap up before the weekend," a Western diplomat said of the extension. "We will not have an agreement before Sunday, so the next few days are about agreeing to terms for a rollover."

Another diplomat said on Wednesday that the decision was still not definite: "We would like it to be on Friday, but a lot needs to happen between now and then. We really do not know yet."

In Washington, US President Barack Obama consulted with Secretary of State John Kerry about the extension. He told reporters after their meeting that Iran had met its commitments under the interim deal but that significant gaps remained.

"Based on consultations with Secretary Kerry and my national security team, it's clear to me that we have made real progress in several areas and that we have a credible way forward," Obama said.

"So, over the next few days, we'll continue consulting with Congress - and our team will continue discussions with Iran and our partners - as we determine whether additional time is necessary to extend our negotiations."

Iranian Foreign Minister Mohammad Javad Zarif said the talks, which resumed two weeks ago in the Austrian capital after five earlier rounds since February, would continue seriously until Sunday.

"We have not yet agreed to the extension of talks," he said after a closed-door meeting with US Deputy Secretary of State William Burns on Wednesday.

The talks, involving the United States, Britain, France, Germany, Russia and China, as well as Iran, have made tangible progress on some issues, but large discrepancies remain especially on the pivotal issue of Iran's enrichment capacity, diplomats said.

An extension of up to six months is theoretically possible according to an interim agreement that Iran and the powers signed in November and began implementing in January.