AMERICAN President Barack Obama has said his government is looking at "all options" to help Iraq fight Islamist militants who have captured the country's second largest city and key towns to the north of Baghdad, but he has ruled out sending ground troops.

Mr Obama's remarks came after the cities of Mosul and Tikrit fell to Sunni Islamist insurgents during a lightning advance that some commentators believe has left Iraq on the brink of civil war.

It is understood the White House may be considering air strikes against the insurgents, who routed Iraqi security forces from Mosul on Monday and have pushed south seemingly unopposed.

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The comments from Mr Obama came 24 hours after UK Foreign Secretary William Hague made clear Britain would not get involved militarily in the struggle.

However, the UK has deployed a humanitarian team in Iraq to assess the needs of civilians fleeing the violent takeover of parts of the country by Islamist extremists.

International Development Secretary Justine Greening said her department stands ready to provide "whatever help is needed" to assist the hundreds of thousands of people believed to have fled their homes in northern Iraq.

The al-Qaeda-inspired group that has led the charge in the area has promised to march on to the capital Baghdad, raising fears about the government's ability to slow the assault following the insurgents' gains.

Fighters from the militant group known as the Islamic State of Iraq and al-Sham (ISIS) captured Saddam Hussein's home town of Tikrit on Tuesday, as soldiers and security forces abandoned their posts and yielded ground once controlled by American forces.

That seizure followed the capture of much of Mosul, Iraq's second largest city, the previous day.

The group and its allies among local tribesmen also hold the city of Fallujah and other pockets of the Sunni-dominated Anbar province to the west of Baghdad.

A spokesman for ISIS said the group had old scores to settle with Prime Minister Nouri al-Maliki's government in Baghdad. The Iraqi leader, a Shiite, is trying to hold on to power after indecisive elections in April.

Mr al-Maliki has called on Iraq's parliament to declare a state of emergency that would give him the "necessary powers" to run the country - something legal experts said could include powers to impose curfews, restrict public movements and censor the media.

The ISIS spokesman, Abu Mohammed al-Adnani, also threatened that the group's fighters would take the southern Iraqi Shiite cities of Karbala and Najaf, which hold two of the holiest shrines for Shiite Muslims.

Mosul, the capital of Ninevah province, and the neighbouring Sunni-dominated province of Anbar share a long border with Syria, where ISIS is also active.

Mosul's fall was a heavy defeat for Mr al-Maliki.

His Shiite-dominated political bloc came first in parliamentary elections in April - the first since the US military withdrawal in 2011 - but failed to gain a majority, forcing him to try to build a governing coalition.

A parliamentary vote to grant Mr al-Maliki emergency powers was delayed yesterday after MPs failed to turn up. Just 128 out of the 325 MPs were present for the vote.

The UN Security Council has denounced the fall of Mosul "in the strongest terms" and has condemned terrorist attacks elsewhere in Iraq aimed at destabilising the country and region.

The council reaffirmed its support for the Iraqi people and government and reiterated that "no act of violence or terrorism can reverse a path towards peace, democracy and reconstruction in Iraq".