America is to supply weapons directly to Kurdish fighters in Iraq for the first time, to help the lightly armed forces in the north push back jihadist rebels, assisted by US air power.
The move follows a change of heart by the Baghdad Government, which had previously objected to arms being sent to Kurdish forces, known as the Peshmerga, which were separate to the official Iraqi Army.
If follows several days of "an unprecedented level of co-operation" between the Iraqi and the Kurdish forces, the US State Department said.
"They're helping each other out," said a spokesman.
"So any way we can get the very urgently needed arms to the Kurds we are actively working on. We'll work with the Government of Iraq to do that, but we believe again there is such an urgent situation that we need to do this."
Over the weekend, the Kurdish Peshmerga beat back last week's lightning advance of Isis (Islamic State) jihadists, assisted by US air support flying from the carrier USS George H W Bush, based in the Gulf.
Since the advance of the jihadists in June, US officials have increased weapons shipments to Baghdad, which is now sharing them with the Kurds. Kurdish offensive capabilities have also been supplemented by Iraqi military helicopters now deploying from Erbil airport and a lot of "inherited" material. The news came as Lieutenant General William Mayville, the joint staff director of operations at the Pentagon, said the strikes were "unlikely to affect" the jihadists' overall capabilities.
The Pentagon said it was now conducting about "50 or 60" sorties a day, including reconnaissance, humanitarian drops and offensive strikes.
There have been 15 separate air strikes since President Barack Obama authorised the use of force on Friday.
Around Sinjar, the dissipation of Islamic State forces into local populations had limited the effectiveness of air strikes as a tool to break the seige of the mountain where many thousands of Yazidi remain stranded.
The US also said it would deploy a disaster response team to help distribute aid to those who have fled the Islamists.
Iraq's President named a leading Shia politician as the new Prime Minister yesterday, driving Nouri al-Maliki from office after eight years.
The nomination of Haider al-Abadi, currently deputy Parliament Speaker, came hours after a defiant Maliki ordered tanks and special forces to take up strategic points around Baghdad. As armour surrounded the Green Zone, where Baghdad's government offices are based, and troops locked down the presidential palace, Maliki gave a surprise television address denouncing President Fuad Masum and threatening legal action for not choosing him as the nominee. Maliki had ignored repeated calls both from domestic opponents and from the US to step down. Critics accuse him of aggravating the crisis by practising sectarian politics.
Washington welcomed the nomination of Abadi and warned that efforts by Maliki to stay in power risked destabilising Iraq further, and would also jeopardise international support for Baghdad. Obama called Abadi's nomination a "promising step forward" and urged "all Iraqi political leaders to work peacefully through the political process".
Abadi, who as an opponent of Saddam Hussein spent years living in exile in Britain, is a member of Maliki's party, but was nominated after a coalition of Shia political factions turned its back on the former Prime Minister. He has 30 days to form a new government.
- additional reporting AP