Washington said Monday it might use a critical fifth round of nuclear talks between Iran and world powers to discuss with Tehran possible cooperation on tackling the crisis raging in Iraq.

The United States and Iran, which have been bitter foes for more than 30 years, are both deeply concerned by a major insurgency by Sunni militants who have overrun swathes of Iraq over the past week.

A senior US official said that as a result "there may be some conversations" with Iranian negotiators on the sidelines of nuclear talks between Iran and six world powers in Vienna that began on Monday.

Washington stressed however that it would push Shiite-majority Iran "to address problems in a non-sectarian way," State Department spokeswoman Jen Psaki said.

While signalling its readiness to talk to Iran on the issue, Washington also ruled out consulting with Tehran on any potential military action.

Pentagon spokesman Rear Admiral John Kirby said "there is absolutely no intention, no plan to coordinate military activities between the United States and Iran."

In the Austria capital were US Deputy Secretary of State William Burns, who held secret nuclear talks with Iran in 2013, and Iranian Foreign Minister Mohammad Javad Zarif.

Zarif was a key interlocutor between Shiite Iran and the US government after the September 11, 2001, attacks, when both sides were keen to oust the hardline Sunni Muslim Taliban in Afghanistan.

"The US and Iran discussed Afghanistan ... so from time to time there have been times where it makes sense to be part of a conversation," the US official said.

US Secretary of State John Kerry told Yahoo News on Monday that he would be open to cooperating with Iran over Iraq, saying he "wouldn't rule out anything that would be constructive".

Iran's chief negotiator, Abbas Araqchi, said beforehand however that negotiators in Vienna would "only discuss Iran's nuclear issue".

- Nuclear focus -

The main focus in Vienna remains however efforts towards a nuclear deal with only five weeks before a July 20 deadline to sign on the dotted line.

The five permanent members of the UN Security Council plus Germany want Tehran to scale back its nuclear activities, while Iran wants all UN and Western sanctions to be lifted.

This long hoped-for accord would aim to once and for all silence fears that Tehran might develop nuclear weapons, and avert a slide into international conflict.

Both sides caution that there is a long way to go as negotiators confront the same sticking points that have dogged diplomatic efforts for the past decade.

The senior US official said however that contrary to the general assessment by experts, both sides actually began to draft a deal at their last meeting in May.

"A little bit of that was done the last time, and it was expected more will take place during this round," the official said.

She added that in US-Iranian bilateral talks last week, both sides "not only understood each other better ... but I think we both can see places where we might be able to close the gaps."

- Thorny issues -

The many thorny issues to be resolved in what would be a fiendishly complex deal include the duration of the mooted accord and the pace of sanctions relief.

But the gorilla in the room remains uranium enrichment, a process that can produce nuclear fuel but also, when highly purified, the core of an atomic bomb.

Iran wants to massively increase the number of centrifuges -- the machines that enrich -- saying it needs them to produce the fuel for a future set of civilian nuclear plants.

The West says these are years if not decades away from being built, fearing that Iran's real aim is to use its centrifuges to enrich uranium to weapons-grade -- which Tehran denies.

French Foreign Minister Laurent Fabius said last week that the West wants Iran to slash the number of centrifuges to "several hundred" from the current 20,000, of which 10,000 are operating.

"We are not even in the same ballpark," Fabius said.

- Extra time -

Under an interim deal struck in November, Iran agreed to freeze certain nuclear activities for six months in return for minor sanctions relief.

This comes to an end on July 20 but it can be renewed -- if both sides agree. Experts say such an extension is probably already under discussion.

The senior US official however denied this.

US President Barack Obama would much prefer to get a deal by July 20 in order to fend off accusations that Iran is merely buying time ahead of midterm US elections in November.

"It will be in the interest of everyone if a deal is signed in the next five weeks," Iranian President Hassan Rouhani said Saturday.