Global powers wrestling to hammer out a ground-breaking deal with Iran to curb its nuclear ambitions are moving complex talks into high gear with a "critical" three weeks left for an accord.

The main players -- US Secretary of State John Kerry, his Iranian counterpart Mohammad Javad Zarif and outgoing EU foreign policy chief Catherine Ashton -- will crisscross the globe ahead of the November 24 deadline seeking to narrow the gaps.

Ashton will first meet in Vienna on November 7 with political directors from the so-called P5+1 grouping -- Britain, China, France, Russia and the United States as well as Germany -- her spokesman Michael Mann said.

She will then fly to Oman to meet with Kerry and Zarif in closed meetings, in the country that first hosted secret talks between old foes Iran and the United States.

Those meetings between the two nations, which still do not have diplomatic ties, are credited with bringing Tehran back to the stop-start negotiations.

Kerry has warned the coming weeks will finally reveal whether the Islamic Republic is truly prepared to make the tough decisions needed to curb its suspect nuclear program and win a lifting of international sanctions.

"We have critical weeks ahead of us," Kerry told PBS television.

"The stakes for the world are enormous. I hope the Iranians will not get stuck in a tree of their own making, on one demand or another, in order to try to find a way together.

"I'm hopeful, but it's a very tough negotiation."

Any historic accord would aim to ease fears, after a decade of rising tensions, that Tehran might develop nuclear weapons under the guise of its civilian atomic program.

In return for reducing the scope of its activities and allowing closer UN inspections, Iran, which denies wanting the bomb, wants painful UN and Western sanctions lifted which have crippled its economy.

Months of intense negotiations, including between Kerry and Zarif in mid-October in Vienna, have made some progress but appear deadlocked on the key issues of uranium enrichment and the pace of any sanctions relief.

Enrichment renders uranium suitable for nuclear power generation and other peaceful uses but also, at high purities, for a nuclear bomb.

- Extension? -

Kerry said his meeting in Oman would be "beginning a slog of going into the last two weeks."

After their Oman talks, Ashton will travel back to Vienna for P5+1 meetings starting on November 18.

Kerry suggested both he and Zarif would also return to the Austrian capital, which has been hosting the negotiations in recent months, for the last stretch.

"We'll be in Vienna for the final days with the P5 plus 1, all of us together trying to come to some kind of an agreement," he told PBS.

There is growing speculation about whether the talks will be extended again, after an earlier July deadline was missed.

Washington has so far insisted that US negotiators are focused on the November 24 date, and have not yet discussed an extension.

Under the interim deal reached in late November 2013, Iran agreed to halt most of its enrichment activities in return for an injection of around $7 billion in frozen oil revenues in bank accounts around the world.

Daryl Kimball, executive director of the Arms Control Association, said he did not believe the interim deal would be extended again.

He predicted though that negotiators may announce they have made substantial progress "but need more time to work out the technical details."

"I think the P5+1 believe, and I think the Iranians believe, that if there is going to be a comprehensive multi-year agreement the decisions necessary to reach that agreement can and should be made now," he told a forum this week.