Washington responded to the sweeping unrest by deploying an aircraft carrier to the Gulf, but Iran has warned against foreign military intervention in its Shiite neighbour, voicing confidence that Baghdad is able to repel the onslaught.

The militants, spearheaded by the powerful Islamic State of Iraq and the Levant (ISIL) jihadist group, have overrun all of one province and chunks of three more since they launched their offensive late on Monday.

Security forces have generally performed poorly, with some abandoning their vehicles and positions and discarding their uniforms, though they seem to have begun to recover from the initial onslaught and have started to regain ground.

Iraqi commanders have said security forces were now starting to push the militants back, and that soldiers had recaptured two towns north of Baghdad.

They will be joined by a flood of volunteers, urged on by a call to arms from top Shiite cleric Grand Ayatollah Ali al-Sistani.

A recruitment centre for such volunteers at the town of Khales in central Iraq came under mortar attack on Sunday, leaving six people dead, including three Iraqi soldiers, police and a doctor said.

US President Barack Obama said he was "looking at all the options" to halt the offensive that has brought the militants within 50 miles (80 kilometres) of Baghdad's city limits, but ruled out any return of US combat troops.

Washington has also ordered aircraft carrier USS George H.W. Bush into the Gulf in response to the crisis.

- Obama under fire -

Obama has been under mounting fire from his Republican opponents over the swift collapse of Iraq's security forces, which Washington spent billions of dollars training and equipping before pulling out its own troops in late-2011.

Iran meanwhile warned on Sunday that "any foreign military intervention in Iraq" would only complicate the crisis, while Germany warned of a potential "proxy war" in the region.

"Iraq has the capacity and necessary preparations for the fight against terrorism and extremism," foreign ministry spokesman Marzieh Afkham was quoted as saying by the ISNA news agency.

"Any action that complicates the situation in Iraq is not in the interests of the country nor of the region," Afkham said, adding, "The people and government of Iraq will be able to neutralise this conspiracy."

Iranian President Hassan Rouhani said a day earlier that Iran had not been asked for help by its neighbour.

But in surprise comments he added that Iran may consider cooperating with its arch-foe the United States to fight the militants in Iraq.

"If we see that the United States takes action against terrorist groups in Iraq, then one can think about it," he said, despite the lack of diplomatic relations between Tehran and Washington for more than three decades.

Brahimi, the former UN and Arab League envoy to Syria, told AFP the international community's negligence of the conflict in neighbouring Syria had precipitated the crisis in Iraq.

"It is a well known rule: a conflict of this kind (in Syria) cannot stay confined within the borders of one country," said Brahimi.

The international community "unfortunately neglected the Syrian problem and did not help to resolve it. This is the result," said Brahimi, who resigned from his post as UN-Arab League representative to Syria in May.

- Grisly scenes -

As Iraq troops began to drive back the militants, they found grisly scenes, amid reports that the militants had carried out summary executions of Iraqi security forces members they captured.

Troops found the burned bodies of 12 policemen as they recaptured the town of Ishaqi in Salaheddin province from the insurgents, a police colonel and a doctor said.

Photos posted online were also said to show militants summarily executing dozens of captured members of the security forces in the province.

The situation on the ground, however, has been further complicated as forces from the autonomous Kurdish region have made territorial advances.

A senior official said on Sunday that Kurdish peshmerga forces had taken control of one of two official border crossings with Syria earlier in the week.

Kurdish forces have also seized the disputed ethnically-mixed northern city of Kirkuk and surrounding areas, as well as other areas.

Amid the confusion, Iraq launched an air strike on a convoy of Kurdish forces Saturday night near Khanaqin, one of the areas of eastern Iraq that Kurds have moved in to, killing six people.

It was not immediately clear if the attack was specifically targeting the Kurdish troops or a case of mistaken identity.

And though violence has dropped off in Baghdad, apparently as militants have concentrated their efforts elsewhere, the capital has not been spared, with a bombing on Sunday afternoon killing nine people.

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