MILiTANTS and pro-Iraqi ­Government forces were locked in fierce battles for a major oil refinery and airfield last night as loyalist troops massed north of Baghdad to strike back at the Sunni Islamists whose drive toward the capital prompted the US to send military advisers to stiffen government resistance.

Baiji, the country's biggest oil refinery, was reportedly surrounded and 180 miles to the north fighting was raging at Tal Afar airport near Mosul.

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

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Pressure is growing on Mr Maliki after Iraq's Grand Ayatollah Ali Sistani yesterday called for Iraq's newly elected parliament to begin work and start the process of ­forming the next government ­without delay.

Mr Sistani's sermon, read by his representative at the Imam Hussein shrine in Kerbala, called for parliament to convene after Iraq's federal court ratified the results this week.

Mr Maliki's Shi'ite bloc won the most seats but, with stalemate among Shi'ite, 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, Mr Maliki has irritated Washington with 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-Shi'ite zealots taking over Iraq.

Mr Obama has stopped short of calling for Mr Maliki to step down, saying: "It's not our job to choose Iraq's leaders." But in what appeared to be a pointed warning to Mr Maliki, he said: "Only leaders that can govern with an inclusive agenda are going to be able to truly bring the Iraqi people together and help them through this crisis."

Near Samarra, 60 miles north of Baghdad and now a frontline in the battle with the Islamic State of Iraq and the Levant (Isis), the provincial governor, a Sunni supporter of Mr Maliki, told troops they would now force Isis and its allies back.

A source close to Mr Maliki said the government planned to hit back now that it had halted the advance that saw Isis seize the main ­northern city of Mosul 11 days ago and sweep down along the Sunni-populated Tigris valley toward Baghdad.

Governor Abdullah al Jibouri, whose provincial capital Tikrit was overrun last week, told soldiers in Ishaqi, just south of Samarra: "Today we are coming in the direction of Tikrit, Sharqat and Nineveh. These troops will not stop," adding that government forces around Samarra numbered more than 50,000.

Yesterday a car bomb in Syria's western Hama province killed 34 and wounded more than 50 people, in an attack blamed on rebels fighting forces loyal to President Bashar al Assad. It was unclear whether the bombing was related to Isis.

The attack came as a video was released by Isis showing five Islamist fighters identified as Australian and British calling on Muslims to join the war in Syria.

The footage showed the five ­militants sitting in a rural area and holding rifles with al Qaeda's flag flying behind them. "We are a State who is implementing the sharia [Islamic law] in both Iraq and Sham [Syria], and look at the soldiers - we understand no borders," said one fighter named as Abu Muthanna al Yemeni from Britain.

The other militants were identified as Abu Bara al Hindi and Abu Dujana al Hindi, also from the UK, and Abu Yahya al Shami and Abu Nour al Iraqi, both from Australia.