The United States said Monday it is open to discussions with Iran on the crisis in Iraq but ruled out consulting with Tehran on any potential military action.

President Barack Obama's administration appeared to back off of earlier comments from Secretary of State John Kerry, who had suggested in an interview that Washington might be willing to consider military cooperation with Tehran to counter the onslaught of Sunni extremists in Iraq.

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

State Department spokeswoman Jen Pskai wrote in a tweet: "To be clear: Open to political conversation with Iran re threat from ISIL (Islamic State of Iraq and the Levant), not military cooperation."

US and Iranian diplomats might address the situation in Iraq on the margins of negotiations on Tehran's nuclear program, which will take place this week in Vienna, Kirby told reporters.

"It's possible on the sideline of those discussions, there could be discussions surrounding the situation in Iraq," he said.

"It's not without precedent that we speak about security issues with Iran. There were discussions about Afghanistan with Iran in the not too distant past," said Kirby, an apparent reference to talks with Tehran on Afghanistan after the attacks of September 11, 2001.

But he added: "There are no plans to consult Iran about military activities inside Iraq."

Kirby said the United States has encouraged Iran and other neighboring countries to "play a constructive role" and respect Iraq's "territorial sovereignty."

In an interview with Yahoo News, Kerry made headlines and triggered speculation with his remarks about potential cooperation with Iran.

When asked about potential US-Iran military cooperation, Kerry said: "At this moment, I think we need to go step by step and see what, in fact, might be a reality, but I wouldn't rule out anything that would be constructive to providing real stability..."

Kerry also expressed caution, saying time would tell what Iran would be ready to do on behalf of its allies in the Shiite-led government in Baghdad.

"Let's see what Iran might or might not be willing to do before we start making any pronouncements," Kerry said.

"I think we are open to any constructive process here that could minimize the violence, hold Iraq together, the integrity of the country, and eliminate the presence of outside terrorist forces that are ripping it apart."

The lightning advance of extremists from the Islamic State of Iraq and the Levant across Iraq, including the capture of Mosul, has alarmed leaders in Tehran and Washington.

Both governments, for their own reasons, oppose the rise of the Sunni jihadists and have a common interest in seeing the Baghdad government fend off the Al-Qaeda inspired forces.

ISIL's offensive has raised fears of a new sectarian civil war between Sunnis and Shiites in the country from which US forces withdrew in December 2011.

The talks on Iran's nuclear program this week in Vienna will include two senior US and Iranian diplomats who have overseen previous sensitive discussions between the two governments.

Iranian Foreign Minister Mohammad Javad Zarif, who will take part in the talks in Vienna, was a pivotal figure between Tehran and Washington after the 9/11 attacks, when both countries shared an interest in ousting the hardline Sunni Muslim Taliban regime in Kabul.

Kerry's deputy, Bill Burns, is also attending the Vienna negotiations. Burns led months of secret talks with Iran in Oman that helped expedite the nuclear discussions.