The sacking of dozens of Iraqi army commanders represents a positive sign that the Baghdad government is reforming its army and reaching out to alienated Sunnis, US Defense Secretary Chuck Hagel said Thursday.

With the US-led air war against Islamic State jihadists relying heavily on Iraqi security forces, Hagel praised moves by the new defense minister, Khaled al-Obaidi, to shake up the top tier of the army.

"This new minister of defense and this new government is reconstituting the leadership of the Iraqi security forces," Hagel told the House Armed Services Committee.

He cited Baghdad's announcement Wednesday that 36 army officers had been sacked, saying the move represented needed changes "across the top" of the army.

"Those are fundamental changes," he said.

Hagel said the defense minister was "moving" to create a national guard that would empower Sunni tribes in western Anbar province, where the IS group has exploited local resentment of the Shiite-led government in Baghdad.

The Pentagon chief also said the new Prime Minister Haidar al-Abadi had taken "steps in the right direction" to reach out to the Sunni population, but that more political reform was required.

Hagel faced tough questioning from lawmakers about the war strategy with some voicing doubts whether the Iraqi forces were up to the task of rolling back the IS militants, after Washington's previous attempt to train the army ended in failure.

US-trained Iraqi units collapsed in June in the face of an IS offensive in the country's second city of Mosul, with army soldiers in some cases throwing down their weapons in a hasty retreat.

- 'We can recover' -

General Martin Dempsey, chairman of the Joint Chiefs of Staff, told the same hearing the Iraqi army had unraveled because of "corrupt leadership" and a sentiment that the IS group was "unstoppable."

US and coalition air strikes had stopped the IS group's advance and "we can recover from the shortcomings they (the Iraqi army) exhibited," he said.

It was unclear who would take up the senior positions in the Iraq army after the sackings, Dempsey said.

"We'll see here very shortly actually who takes the place of those who have been changed," the general said.

But Dempsey, who led US troops in Iraq during the 2003-2011 occupation, warned of dire consequences if the Baghdad government failed to follow through on promises to bring Sunnis and Kurds back into the fold.

"That's why I've said that one of the important assumptions about this campaign is that the Iraqi government does establish its intent to create a government of national unity.

"I can predict for you right now, if that doesn't happen, then the Iraqi security forces will not hold together," he said.

Thursday's hearing marked the first time Hagel and Dempsey faced lawmakers since President Barack Obama last week approved an additional 1,500 troops to deploy to Iraq to advise and train local forces, expanding the military presence to a total of 3,100 American forces.