Britain 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 has announced.
Ms Greening said the UK's Department for International Development 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.
Foreign Secretary William Hague has made clear that Britain will not get involved militarily in the struggle between the administration of prime minister Nouri al-Maliki and jihadists from the Islamic State of Iraq and the Levant (Isis) group.
The al Qaida splinter group took control this week of Iraq's second city Mosul and Saddam Hussein's home city of Tikrit as part of an effort to set up a Sunni militant enclave across the Iraq/Syria border.
An Isis spokesman said they would now march on Baghdad to settle scores with Maliki's Shi'ite-led government. Abu Mohammed al-Adnani also threatened that Isis fighters will take the southern cities of Karbala and Najaf, which hold two of the holiest shrines for Shi'ite Muslims.
Militants have taken 48 hostages from the Turkish consulate in Mosul and United Nations secretary general Ban Ki-moon urged the international community to "unite in showing solidarity with Iraq as it confronts this serious security challenge".
Mr Hague said it was primarily the responsibility of the Iraqi government to deal with the crisis.
The Foreign Secretary told the BBC: "Britain won't be getting involved militarily in this situation. We are considering if we need to send humanitarian aid. We are very concerned about the hundreds of thousands of people who have been displaced. With our very large humanitarian budget, we may be in a position to assist with that and we are looking at that now.
"But we will not be getting involved militarily. We will support the United States in anything they decide to do. We are in consultation with them.
"But I stress that it is for the Iraqi leadership primarily to respond to this. This is a democratic country with an elected government with considerable resources and the prime responsibility rests with them in their own country to deal with this issue."
Ms Greening revealed that UK experts were on the ground assessing the needs of those caught up in the fighting.
"Hundreds of thousands of people, including vulnerable women and children, are being forced to flee their homes as fighting spreads across Northern Iraq," said the International Development Secretary.
"Last night I deployed a team of humanitarian experts to assess the situation on the ground and coordinate with our partners. We are monitoring the situation very closely and stand ready to provide whatever help is needed."
Deputy Prime Minister Nick Clegg made clear he would resist any call to send troops to the country, where the UK took part in a US-led invasion to oust dictator Saddam in 2003
"I don't think we should go back into Iraq," the Liberal Democrat leader told his regular Call Clegg phone-in show on LBC 97.3 radio.
Mr Clegg said some people were asking "if the invasion of Iraq in the first place has contributed to the instability in Iraq and, more widely, should we now be going back in to try and sort it?"
But he added: "I don't think having made one mistake, you repeat it by making a second one."
An Iraq-born Conservative MP blamed the West's failure to intervene in Syria for helping to fuel the upsurge of Islamist unrest in neighbouring Iraq.
Nadhim Zahawi blamed US "de-Ba'athification" policy in the wake of the 2003 invasion for sowing the seeds of the insurgency by disbanding the Iraqi army, leaving hundreds of thousands of armed and trained Sunni men with no jobs and no prospects.
But he said the "divisive sectarian" rule of the Maliki regime and the lengthy and increasingly extremist-led civil war in Syria were fanning the flames.
The MP - who was among those who supported strikes on the Syrian regime of Bashar Assad last summer which were effectively voted down by the Commons - said failure to act there had opened the door to groups such as Isis.
"I argued for very limited intervention to take the chemical weapons out of the game in Syria to protect the silent majority of innocent people," he told BBC Radio 4's Today programme.
"Early intervention in Syria, supporting people who were working towards replacing Bashar al Assad with a moderate democracy which is inclusive would have been the right move because otherwise the message you send to the silent majority is actually you don't care about them and that is where these groups thrive because they go in and they are brutal and they take over and fill that vacuum."
Mr Clegg said Britain should "do battle with the ideologies that are driving this" and stand up for the values of "democracy, politics over violence and equality before the law", but could not act unilaterally.
"The one lesson we must have learnt over and over again over the last several years is that in the modern world even great superpowers like America can't go round unilaterally fixing stuff. We have to work together," said the Deputy Prime Minister
"That is why I think the role of the United Nations and other multilateral organisations to bring various parts of the world together is the only way we can respond internationally."
Asked how the Government would treat any request for military equipment and supplies from the Maliki administration, Mr Clegg said: "There are very, very strict guidelines governing how we respond to these kinds of requests and they would be treated in the normal way.
"We provide help to the Iraqi government, we provide training and equipment and so on. I have no idea what the Iraqi government will be asking of others in terms of the material support or training they need."
The RAC motoring organisation warned that the unrest in Iraq is likely to force petrol prices up at pumps in the UK by 2p a litre in the short term, and more if the crisis continues.
RAC head of external affairs Pete Williams said: "The worsening situation in Iraq is causing a knee-jerk reaction in the global fuel market with wholesale prices going up one pence over Wednesday and Thursday.
"This uncertainty is being made worse as the warring factions advance on some of the oil facilities, particularly the city of Kirkuk. We anticipate that it is likely to push the pump price of both petrol and diesel by 2p per litre in the short-term and this could well go much further.
"After a period of relative stability, and a welcome drop in the cost differential for diesel, this once again demonstrates how UK motorists and businesses are at the mercy of world events and the volatile nature of the global fuel market."
The Prime Minister's spokesman said the Government was monitoring whether any Britons had gone to join the insurgency in Iraq.
"Our security services and all the relevant agencies will be monitoring those types of risks very closely. Clearly there is a very porous border between parts of Syria and parts of Iraq," the spokesman said.
"As the Prime Minister has said, the greatest extremist activity and jihadist threats to the international community are in Syria. We need to keep these things under very close watch."
Shadow foreign secretary Douglas Alexander said: "The situation in northern Iraq remains extremely serious, and the violence has already forced hundreds of thousands of innocent people to flee their homes.
"The United Nations Security Council was right to condemn these attacks and to call for international solidarity with Iraq's leaders as they confront this serious security challenge.
"The UK's decision to increase its support for the immediate Iraqi humanitarian response is important, and the international community must continue to monitor the situation closely.
"Iraq has the ongoing support of the international community in its efforts to combat terrorism, but the authorities there must now do all they can to protect civilians from further attacks.
"Iraq's leaders must now come together quickly to form a new government and act to address the violent and instability which risks spreading further unless appropriate action is now taken by the Iraqi authorities."