A young Iraqi boy, wearing an army uniform, sits near Muslim worshippers loyal to Shiite cleric Moqtada al-Sadr performing the Friday prayer in the mainly Shiite Sadr district in Baghdad on Friday. Photo: AFP

Iraqi forces were massing north of Baghdad on Friday, aiming to strike back at Sunni Islamists heading toward the capital, which has prompted the US to offer to send military advisers to reinforce the government defenses.

US President Barack Obama offered up to 300 Americans to help coordinate the fight. But he held off granting a request for airstrikes from the Shiite-led government and renewed a call for Prime Minister Nuri al-Maliki to do more to overcome sectarian divisions that have fueled resentment among the Sunni minority.

Speculation that Maliki might be forced aside was heightened when the country's senior Shiite cleric urged a speedy formation of a new government following the ratification this week of the results of a parliamentary election held in April.

Maliki's Shiite bloc won the most seats but with stalemate among Shiite, Sunni and ethnic Kurdish groups, the new assembly has yet to sit. Grand Ayatollah Ali al-Sistani wrote in a Friday sermon that a constitutional deadline for convening to choose a new prime minister and government should be respected.

In office since 2006, Maliki has irritated Washington by the way he has alienated Sunnis, and there has been speculation he has also lost the confidence of allies in Iran as Tehran and the US look to end decades of mutual hostility to prevent anti-Western, anti-Shiite zealots from taking over Iraq.

In the area around Samarra, on the main highway 100 kilometers north of Baghdad, which has become a frontline of the battle with the Islamic State of Iraq and Syria (ISIS), the provincial governor, a rare Sunni supporter of Maliki, told cheering troops they would now force ISIS and its allies back.

A source close to Maliki told Reuters that the government planned to hit back now that it had halted the advance which saw ISIS seize the main northern city of Mosul, capital of Nineveh province, 10 days ago and sweep down along the Sunni-populated Tigris valley toward Baghdad as the US-trained army crumbled.

Governor Abdullah al-Jibouri, whose provincial capital Tikrit was overrun last week, was shown on television on Friday telling soldiers in Ishaqi, just south of Samarra: "Today we are coming in the direction of Tikrit, Sharqat and Nineveh."

"These troops will not stop," he added, saying government forces around Samarra numbered more than 50,000.

The participation of Shiite militias and tens of thousands of new Shiite army volunteers has allowed the Iraqi military to rebound, after mass desertions by soldiers last week allowed ISIS to carve out territory where it aims to found an Islamic caliphate straddling the Iraqi-Syrian border.

"The strategy has been for the last few days to have a new defense line to stop the advance of ISIS," a close ally of Maliki told Reuters.

"We succeeded in blunting the advance and now are trying to get back areas unnecessarily lost," said the source.

Pockets of fighting continue. Government forces appeared to be still holding out in the sprawling Baiji oil refinery, the country's largest, 100 kilometers north of Samarra, residents said.