Western warplanes dropped desperately needed aid on Sunday to an Iraqi Shiite town under blockade by jihadists for well over two months as preparations to break the siege dragged on.
The aid drops were accompanied by US air strikes and were the furthest south that US forces have intervened in Iraq, barring reconnaissance flights, since their withdrawal in December 2011.
The mainly Turkmen residents of the Salaheddin province town of Amerli, where the United Nations has warned of the risk of sectarian massacre by the besieging Sunni Arab extremists, have been running desperately short of food and medicines.
Thousands of Shiite militiamen and Kurdish fighters have been massing for days for an operation to break the siege, alongside the Iraqi army, which was left in disarray by the lightning offensive the jihadists launched in second city Mosul in early June.
Washington launched air strikes in support of Kurdish forces in northern Iraq on August 8.
But it has so far been reluctant to expand its operations amid Pentagon warnings that military intervention alongside Baghdad government forces risks further alienating Sunni Arabs without more strenuous efforts by the Shiite-led administration to engage the disenchanted minority community.
Shiite militia and Kurdish forces closed on the besieged enclave on Sunday, reaching within five kilometres (three miles) of the forces inside it, militia commander Mohammed Mahdi al-Bayati said.
A roadside bomb south of the town killed four militiamen, officers said.
Australian, British, French and US aircraft dropped relief supplies to the thousands of civilians trapped in the enclave.
"At the request of the government of Iraq, the United States military today airdropped humanitarian aid to the town of Amerli," said Pentagon spokesman Rear Admiral John Kirby.
"The United States Air Force delivered this aid alongside aircraft from Australia, France and the United Kingdom, who also dropped much needed supplies."
The aid drops came alongside "coordinated air strikes against nearby ISIL terrorists in order to support this humanitarian assistance operation," he added.
He was referring to the jihadist forces who called themselves the Islamic State in Iraq and the Levant (ISIL) before dropping the geographic reference when they declared a pan-Islamic caliphate after their sweeping advance in June.
"The operations will be limited in their scope and duration as necessary to address this emerging humanitarian crisis and protect the civilians trapped in Amerli," Kirby said.
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- US air strikes -
US Central Command said the US supplies dropped included around 47,775 litres (10,500 gallons) of drinking water and 7,000 pre-packaged meals.
Three US air strikes destroyed "three ISIL Humvees, one ISIL armed vehicle, one ISIL checkpoint and one ISIL tank near Amerli".
"The US military will continue to assess the effectiveness of these operations and work with the Department of State, the US Agency for International Development, as well as international partners including the government of Iraq, the United Nations, and non-government organisations to provide humanitarian assistance in Iraq as needed," Kirby said.
The US military also launched fresh air strikes Saturday on IS forces near Iraq's largest dam, north of the militant-held northern city of Mosul, the Pentagon said.
Kurdish forces retook the dam after briefly losing it to the jihadists earlier this month, securing the source of much of the power and irrigation water for the region around Iraq's second city.
The jihadist Islamic State and its allies control swathes of both northern and western Iraq and neighbouring northeastern Syria where their rule has witnessed a spate of atrocities that have shocked the world.
Washington has thus far ruled out any cooperation with the Damascus regime against the jihadists but has attempted to enlist the support longtime foe Tehran, a key backer of Syrian President Bashar al-Assad.
Writing in the New York Times, US Secretary of State John Kerry urged "a united response led by the United States and the broadest possible coalition of nations" to combat IS.
Kerry said he and Defence Secretary Chuck Hagel would meet European counterparts on the sidelines of an upcoming NATO summit to enlist assistance, and then travel to the Middle East to build support "among the countries that are most directly threatened".
US President Barack Obama has acknowledged that Washington has no strategy yet to tackle IS, which has declared an Islamic "caliphate" in the territory under its control in Iraq and Syria.