Britons have been warned to leave Irbil, the capital of Kurdish Iraq, as Islamist fighters move towards the city.
The move comes as UK Defence Secretary Michael Fallon said Britain was offering help to the Americans in their efforts to provide humanitarian support to people trapped by the advance.
British Airways has suspended its flights over the war-torn region, after the US Federal Aviation Administration (FAA) issued a notice "restricting US operators" from flying over Iraqi airspace.
About 50,000 members of the Yazidi community, including many women and children, have been forced to flee into the Sinjar mountains in northwest Iraq following threats from fighters from Islamic State (IS) - previously known as ISIS or ISIL.
IS militants have moved across northern Iraq and are also said to be threatening Kurdish territory.
Christian refugees who have fled Irbil, which is now threatened by IS The Foreign Office advised against all travel to areas hit by recent fighting and said Britons already in Kurdistan, including those in Irbil, should remove themselves from areas nearby.
A statement outlining the updated travel advice said: "The FCO advise against all travel to Anbar, Ninewah, Salah-ad-Din and Diyala provinces and all of the area south of Kirkuk City limits in Tam'mim province.
"The FCO also advise against all travel to those areas of the Kurdistan Region (Irbil, Suleymania and Dohuk provinces)."
Mr Fallon said the UK was ready to offer help to the US operation.
He told Sky News: "We welcome what the Americans are doing, in particular to bring humanitarian relief and to prevent any further suffering.
People gather at an anti-IS protest outside Downing Street in London "But our focus is on assisting that humanitarian mission using our military in support of the Americans, in terms of refuelling and surveillance, to under pin their mission and to add to it with food drops of our own."
His comments came as it emerged that the US had carried out airstrikes to hit IS forces on the ground.
Earlier, David Cameron had insisted the world must help the Yazidi community in their "hour of desperate need".
The Prime Minister said he "utterly" condemned "the barbaric attacks", adding he was "especially concerned" for people trapped on Mount Sinjar.
He added that he fully backed President Obama's decision to press ahead with airstrikes, which he said had been requested by the Iraqi government.
Iraqi Ambassador to the UK Faik Nerwayi told Sky News that he had heard the the airstrikes had already brought a wave of optimism to some of the those who had been forced to flee.
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