Iraqi forces appear to be rallying and bolstering their defence of Baghdad as Sunni rebels sweep across the country's north, the Pentagon says.

"We also have reason to believe - certainly indications - that the Iraqi security forces are stiffening their resistance and their defence and are coalescing, particularly in and around Baghdad, and that's encouraging," Pentagon spokesman Rear Admiral John Kirby told reporters on Tuesday.

With Iraqi troops now receiving help from Shia volunteers, Kirby said "it certainly appears as if they have the will to defend the capital".

Rebel group, the Islamic State of Iraq and the Levant (ISIL), has staged a stunning advance over the past week that has sent the United States-trained Iraqi government army into a retreat.

After taking Mosul, ISIL's fighters captured a large area of mainly Sunni territory stretching towards the capital and overnight briefly held parts of Baquba, the provincial capital of Diyala province 60km north of Baghdad, before being repelled by government forces.

Sackings, court martial for desertion

The US spent millions of dollars over several years training and arming a new Iraqi army after disbanding the Sunni-led force created by the late dictator Saddam Hussein.

Kirby has acknowledged the Pentagon was "certainly disappointed in the performance of some of these units".

Iraqi soldiers and police in some cases fled en masse before the ISIL offensive, abandoning their vehicles and uniforms.

Iraqi Prime Minister Nouri al-Maliki sacked several senior officers over the recent defeats and ordered one of them to face court-martial for desertion.

US President Barack Obama has already deployed about 160 troops to Baghdad to strengthen security for the US embassy and has approximately another 100 forces ready outside of Iraq for various "contingency" operations.

But Kirby said indications that Iraqi troops could be improving their defences did not mean the White House would take more time to weigh potential military action.

Appeal for unity

Earlier on Tuesday Maliki broadcast a joint appeal for national unity with Sunni critics of his Shia-led government.

Hours after Maliki's Shia allies had vowed to boycott any cooperation with the biggest Sunni party, Shia, Sunni and Kurdish leaders met behind closed doors.

They then stood before cameras as Maliki's Shia predecessor Ibrahim al-Jaafari read a statement denouncing "terrorist powers" and supporting Iraqi sovereignty.

"No terrorist powers represent any sect or religion," Jaafari said in the address, which included a broad promise of "reviewing the previous course" of Iraqi politics.

Afterwards, most of the leaders, including Maliki and Osama al-Nujaifi, the Iraqi parliamentary speaker, walked away from each other in silence.

Source: Agencies