Iran and six world powers are working to finalize the terms of a likely extension in negotiations over Tehran’s nuclear program beyond a July 20 deadline and an announcement may come as early as Friday.

Iran and six world powers are working to finalize the terms of a likely extension in negotiations over Tehran's nuclear program beyond a July 20 deadline and 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 decade-long dispute.

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

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

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

Another diplomat said on Wednesday 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, the White House on Wednesday praised Iran for "surprisingly" favorable behavior over the last six months during nuclear talks with Western powers, but it declined to say whether an extension for the talks would be agreed.

White House spokesman Josh Earnest said U.S. Secretary of State John Kerry and President Barack Obama would discuss the various possible paths forward for the Iran talks during a meeting on Wednesday.

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 U.S. 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 gaps remain especially on the pivotal issue of Iran's enrichment capacity, diplomats said.