The US military's top officer said Thursday that Iraqi forces had shored up their defenses against Sunni militants but would be hard-pressed to regain territory without outside help.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, said American advisers were still evaluating the state of the Iraqi army, and he suggested US military action was not imminent.

Dempsey told a news conference that Iraqi forces were not yet in a position to stage a major counter-offensive after being driven back by Sunni extremists in recent weeks.

US military advisers in the capital found that Iraqi security forces are "stiffening, that they're capable of defending Baghdad," Dempsey said.

However, they also found the Iraqi forces "would be challenged to go on the offense, mostly logistically challenged."

"If you are asking me will the Iraqis, at some point, be able to go back on the offensive to recapture the part of Iraq that they've lost... probably not by themselves," he said.

But the Iraqi army's shortcomings did not necessarily mean the United States would have to take military action, he said.

"I'm not suggesting that that's the direction this is headed."

About 200 US military advisers have deployed to Baghdad to assess the state of the Iraqi army and the threat posed by the Islamic State (IS) jihadists, who have seized control in areas north and west of Baghdad.

The advisory teams have set up command centers in Baghdad as well as in Arbil in the north, he said.

Apart from the advisers, nearly 500 US troops have been sent to Iraq to bolster security at the American embassy and parts of the Baghdad airport.

- 'Very different' US role -

Dempsey and Pentagon chief Chuck Hagel both insisted that the US troops in Iraq had no combat mission but left open the possibility that President Barack Obama might opt to order air strikes against the IS extremists in the future.

Dempsey said the role of American troops in Baghdad was "very different" than in the past, when US forces invaded Iraq in 2003 and waged war against insurgents until withdrawing in 2011.

"I mean, assessing, advising and enabling are very different roads than attacking, defeating and disrupting," he said.

That could change if the threat posed by Sunni extremists led the US president to order military action, he said.

"I'm just suggesting to you we're not there yet," said Dempsey, who led combat troops during the US war in Iraq.

The four-star general said that future US military assistance might not require an "industrial-strength effort" to aid the Iraqi forces. Instead, smaller scale aid could be successful if Baghdad could isolate the IS jihadists from other Sunni groups.

He warned that an Iraqi military campaign designed to roll back the Islamist militants would take time to develop and would have to be accompanied by clear signals from the Shiite-led government in Baghdad that it is ready to reach out to Sunni and Kurdish communities.

Dempsey said "the first step in developing that campaign is to determine whether we have a reliable Iraqi partner that is committed to growing their country into something that all Iraqis will be willing to participate in.

"If the answer to that is 'no,' then the future's pretty bleak."

He said the IS extremists had prevailed not through military prowess but by exploiting the Sunni population's distrust and lack of confidence in Prime Minister Nuri al-Maliki's government.

The militants had "bought some people off, they threatened the families of others" and "they reminded everyone that the central government in Iraq was not operating on their behalf," Dempsey said.

The Iraq army "collapsed in the face of a future that didn't hold out any hope for them," he said.

The White House echoed Dempsey's call for unity on Thursday, offering a cool reception to a proposal by Iraq's Kurdish leader for an independence referendum.

The crisis in Iraq has created an unusual situation in which the United States and Iran are both supporting the same side in the conflict, with each lending aid to Baghdad.

Dempsey reiterated that there were no plans for US military cooperation with Tehran but said "it's not impossible that in the future we would... have reason to do so."

He acknowledged that Iran was flying drones over Iraq and that Tehran's activity in Iraq was "more overt" than in the past.