Shiite Iran offered Saturday to consider working with long-time foe Washington if it takes the lead in helping repel Sunni Arab militants who have seized a swathe of northern Iraq.

The offer came as Iraqi commanders said soldiers had recaptured two towns north of Baghdad, thousands of volunteers answered a call to arms from top Shiite cleric Grand Ayatollah Ali al-Sistani, and a US carrier group steamed into Gulf waters.

Prime Minister Nuri al-Maliki visited the besieged city of Samarra north of the capital Friday to rally troops and pray at the Al-Askari Shiite shrine, a revered site whose 2006 bombing by Al-Qaeda sparked a sectarian conflict that killed tens of thousands.

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

"We will not be sending US troops back into combat in Iraq, but I have asked my national security team to prepare a range of other options that could help support Iraqi security forces," he said.

Pentagon spokesman Rear Admiral John Kirby said Defence Secretary Chuck Hagel had ordered aircraft carrier USS George H.W. Bush into the Gulf in response to the crisis.

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 2011.

- Maliki given 'unlimited powers' -

Iranian President Hassan Rouhani, who since taking office last August has overseen a rapprochement with a superpower Tehran long derided as the "Great Satan", said his government was prepared to consider offering help.

"If we see that the United States takes action against terrorist groups in Iraq, then one can think about it," Rouhani told a news conference.

Iraq's Shiite premier said the cabinet had granted him "unlimited powers" to reverse the offensive, in which militants swept towards Baghdad after overrunning second city Mosul on Tuesday before losing some of its steam.

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

It was one of the closest points to Baghdad militants reached as they overran a large part of northern and north-central Iraq.

Troops also retook the nearby Muatassam area of Salaheddin, the colonel said.

Late Friday, police and residents expelled militants from another town in the province, Dhuluiyah, witnesses said.

Security forces have also held fast in the Muqdadiyah area of Diyala province, preventing militants from taking the town in heavy fighting, police said.

In Samarra, reinforcements were awaiting orders to launch a counter-offensive against areas north of the city, including Dur and Tikrit, seized by militants earlier this week, an army colonel said.

North of Baghdad, gunmen attacked a convoy carrying the head of Iraq's anti-corruption watchdog, killing nine police, and a convoy carrying the deputy head of the Shiite religious endowment was also targeted, killing eight guards.

- Starting to regain ground -

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.

They will be joined by a flood of volunteers, urged on by Sistani's call Friday for Iraqis to join up to defend the country.

A representative of Sistani, who is adored by Shiites but rarely appears in public, made the call on his behalf from the shrine city of Karbala, south of Baghdad.

A Qatar-based union of Sunni Muslim clerics on Saturday denounced the call, saying developments in Iraq were a "result of oppression and exclusion of people that wanted freedom".

Obama said while the US was willing to help, Iraq needed to heal the deep divide between the Shiite-led government and the Sunni Arab minority, whose resentment jihadists have exploited.

Washington "will not involve itself in military action in the absence of a political plan by the Iraqis that gives us some assurance that they're prepared to work together," Obama said.

"Any action that we may take to provide assistance to Iraqi security forces has to be joined by a serious and sincere effort by Iraq's leaders to set aside sectarian differences."

State Department deputy spokeswoman Marie Harf dismissed Republican lawmakers' criticism that a residual US force would have prevented the Iraqi army's collapse.

"When we left Iraq, after years of sacrifice and American taxpayer money, and certainly our troops felt that sacrifice more than anyone, the Iraqis had an opportunity," Harf told reporters.

Instead, Iraqi leaders "created a climate where there were vulnerabilities when it came to the cohesion of the Iraqi army," she said.

The militant advance towards Baghdad
Baghdad (AFP) June 13, 2014 - A snapshot of developments in Iraq, where Sunni militants are marching on Baghdad after seizing several provinces.

Tuesday, June 10

- Hundreds of jihadists, mostly from the Islamic State of Iraq and the Levant (ISIL), seize Iraq's second biggest city Mosul in a lightning offensive and take control of Nineveh province. They also overrun sections of the nearby provinces of Kirkuk and Salaheddin.

The swift collapse of Baghdad's control comes after the loss of Fallujah, west of the capital, earlier this year.

- Prime Minister Nuri al-Maliki says the government will provide weapons and equipment to citizens who volunteer to fight against militants and that the cabinet has created a special crisis cell.

- Washington says the jihadists pose "a threat to the entire region".

Wednesday, June 11

- The militants seize the city of Tikrit, Salaheddin's provincial capital. Security forces repel an assault on the highly sensitive city of Samarra, witnesses say.

- ISIL storms the Turkish consulate in Mosul and kidnaps 49 people including the head of the mission, after earlier seizing 31 Turkish truck drivers from a power station.

- ISIL spokesman Abu Mohammed al-Adnani promises that the battle will "rage" on Baghdad and Karbala to its southwest, considered one of the holiest sites for Shiite Muslims.

Thursday, June 12

- Militants seize two strategic areas of Diyala province northeast of Baghdad, after security forces withdraw.

- Kurdish forces take control of the disputed oil hub of Kirkuk to protect it against jihadists. Iraqi Kurds want to incorporate Kirkuk province into their autonomous region, a move Baghdad strongly opposes.

- Iranian President Hassan Rouhani promises Iraq's Maliki his government's full support against "terrorism".

Friday, June 13

- Militants battle pro-government forces near Muqdadiyah, just 80 kilometres (50 miles) from Baghdad's city limits. The government announces a new security plan for the capital.

- Leading Shiite cleric Grand Ayatollah Ali al-Sistani urges Iraqis to take up arms against the militants marching on Baghdad.

- Maliki says security forces have begun clearing cities of "terrorists". Residents of Samarra, just 110 kilometres (70 miles) from Baghdad, say gunmen are gathering to its north, east and southeast.

- US President Barack Obama says he is examining options short of sending ground troops, but warns that Iraq must heal its own divisions.

- UN human rights chief Navi Pillay cites reports of extrajudicial killings and summary executions.

- The International Organisation for Migration estimates that 40,000 people have fled Tikrit and Samarra, adding to half a million people believed to have fled Mosul.