Iraqi forces have regained full control of the country's biggest oil refinery after heavy fighting with Sunni militants attempting to seize it, the authorities claim.

A refinery employee and witnesses said the insurgents led by the jihadist Islamic State in Iraq and the Levant (ISIS) had withdrawn from the sprawling Baiji complex after losing 100 fighters as troops and helicopter gunships repelled repeated attacks, according to the Iraqi military.

The retaking of the plant, north of Baghdad, comes amid calls for the Iraqi prime minister Nouri al Maliki to quit as a condition of US help in driving back insurgents who have seized large swathes of the country.

The administration of Mr al Malaki has requested that America launch airstrikes against the ISIS extremists, whose lightning offensive in the north of the country - including the capture of the city of Mosul - saw the US-bankrolled military crumble.

Speaking at the White House, Barack Obama said the US would be prepared to take targeted military action but that American troops would not return to combat in Iraq.

The president said additional equipment and up to 300 additional military advisers could be provided to help fight the ISIS insurgency.

Mr Obama said it was not for the US to choose Iraq's leaders, but delivered a clear message to Mr al Malaki when he said only leaders "with an inclusive agenda" could lead the country through the crisis.

Earlier, in an interview with Sky News, Nato Secretary General Anders Fogh Rasmussen warned that the government in Baghdad risked splitting the country unless it reached out to all groups.

Speaking in London alongside Mr Rasmussen, David Cameron said there is "no doubt" the Baghdad government has not done enough to heal sectarian divides.

The Prime Minister reiterated that he is taking the threat from British fighters who may join ISIS and later target the UK for terror attacks "extremely seriously".

The advance of ISIS - an al Qaeda breakaway group - has only been slowed by a regrouped army, Shia militias and volunteers, who have signed up to join the battle against the militants.

ISIS has seized a chemical weapons facility built by Saddam Hussein, which contains a stockpile of old weapons, according to the Wall Street Journal.

But US officials are reportedly confident that any weapons stored at the Al Muthanna complex - which has not been in active use for years - are not usable.

A potential obstacle to greater US military involvement is Mr al Maliki, with several leading figures in Congress from across the political divide speaking out against the premier and calling for him to quit.

Dianne Feinstein, the Democrat chair of the Senate Intelligence Committee, said: "The Maliki government, candidly, has got to go if you want any reconciliation."

Republican senator John McCain backed military support but urged Mr Obama to "make it make very clear to Maliki that his time is up".

And General David Petraeus, who led the US troop surge ahead of America pulling out of Iraq, says there should not even be air support without major change in Baghdad.

The former CIA chief warned Washington risked becoming an "air force for Shiite militias", if it agreed to the request for support.

But a spokesman for Mr al Maliki said the PM will not step down, insisting his administration was inclusive, with Sunni Muslims represented in key roles.