An Iraqi civilian, volunteering to fight a militant offensive, kisses a Koran as he queues with comrades in the southern port city of Basra before boarding a bus to reach Mosul on Sunday. Photo: AFP

Iraq said Sunday it had "regained the initiative" against militants who have seized vast swathes of territory, as former UN mediator Lakhdar Brahimi blamed the crisis on global neglect toward Syria's civil war.

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 neighbor, voicing confidence that Baghdad is able to repel the onslaught.

The militants, spearheaded by the powerful Islamic State of Iraq and Syria (ISIS), have overrun all of one province and chunks of three more.

Iraqi commanders have said their forces are now starting to push the militants back, and that soldiers had recaptured two towns north of Baghdad, with a spokesman announcing that Iraqi security personnel had killed 279 "terrorists" in the past 24 hours.

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

For 24 hours, Syria's army has been pounding major bases of the ISIS in coordination with the Baghdad government, a monitor said Sunday.

The strikes against ISIS - which has spearheaded a week-long jihadist offensive in Iraq - have been more intense than ever, said the Syrian Observatory for Human Rights.

"The regime air force has been pounding ISIS's bases, including those in the northern province of Raqa and Hasakeh in the northeast," which borders Iraq, said the Britain-based group.

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

Iranian foreign ministry spokeswoman Marzieh Afkham meanwhile warned on Sunday that "any foreign military intervention in Iraq" would only complicate the crisis.

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

But in surprise comments Rouhani added that Iran may "think about" cooperating with its arch-foe the US to fight the militants in Iraq.

Iraqi 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.

US Secretary of State John Kerry said US assistance to Iraq would only work if Iraqi leaders overcame deep divisions, the State Department said on Saturday.

Kerry spoke with Iraqi Foreign Minister Hoshyar Zebari in a call on Saturday, the State Department said in a statement.

Kerry also urged Iraq to quickly ratify the results of its April 30 parliamentary elections and form a government without the long period of wrangling that followed the 2010 elections.