The United States ordered an aircraft carrier into the Gulf Saturday over the Iraq crisis, where Sunni Arab jihadists have seized a swathe of the country, as Shiite Iran said it would consider helping foe Washington should it take action.

The order came as Iraqi commanders that soldiers had recaptured two towns north of Baghdad, and thousands of volunteers answered a call to arms from top Shiite cleric Grand Ayatollah Ali al-Sistani.

US President Barack Obama said he was "looking at all the options" to halt the offensive that has brought Sunni Arab 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 Defense 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 Nuri al-Maliki 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 steam.

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.

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 the besieged city of Samarra, north of the capital, 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.

Maliki visited Samarra 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.

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.

Former British prime minister Tony Blair, meanwhile, hit out at critics who linked the 2003 invasion of Iraq with the current violence in the country, blaming instead the West's failure to act in Syria.

Blair, who took Britain into the US-led war to remove Saddam Hussein and is now a diplomatic envoy in the Middle East, also criticised the sectarianism of the government in Baghdad.

In a long article published on his website, Blair said arguments that there would be no crisis in the region if the Iraqi dictator had remained in power were "bizarre".

He condemned the sectarianism of Maliki's government, who he said had "snuffed out what was a genuine opportunity to build a cohesive Iraq". (AFP)