BAGHDAD (Dispatches) -- Iraqi forces broke through to the ISIL-besieged Shia town of Amerli Sunday, where thousands of people have been trapped for more than two months with dwindling food and water supplies.

It is the biggest offensive success for the Iraqi government since militants led by the ISIL terrorist group overran large areas of five provinces in June, sweeping security forces aside.

Aircraft from several countries also dropped humanitarian aid to Amerli.

The mainly Shia Turkmen residents of the town in Salaheddin province were running desperately short of food and water, and endangered both because of their Shia faith, and their resistance to the militants, which has drawn harsh retribution elsewhere.

UN Iraq envoy Nickolay Mladenov had warned that they faced a "massacre” by the besieging militants.

"Our forces entered Amerli and broke the siege,” Iraqi security spokesman Lieutenant General Qassem Atta told AFP, an account confirmed by a local official and a fighter from the town.

"It is a very important success,” Atta later said on state television, adding that there was still fighting in the area.

The operation was launched on Saturday after days of preparations in which Iraqi security forces, Shia militiamen and Kurdish fighters deployed for the assault and Iraqi aircraft carried out strikes against militants.

The United States claimed that it carried out three airstrikes in the Amerli area, expanding its air campaign outside the far north for the first time, while Australian, British, French and U.S. aircraft dropped relief supplies for the town but Iraqi officials dismissed the claim.

"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 aid drops came alongside "coordinated air strikes against nearby (ISIL) terrorists in order to support this humanitarian assistance operation”, he added.

Western aid for Amerli was slow in coming, however, with the burden of flying supplies and launching strikes in the area largely falling to Iraq’s fledging air forces.

The ISIL and its allies control swathes of both northern and western Iraq and neighboring northeastern Syria where their rule has witnessed a spate of atrocities that have shocked the world.

Washington has said that operations in Syria will be needed to defeat ISIL, but has so far ruled out any cooperation with the Damascus government against the militants.

It has, however, attempted to enlist the support of long-time foe Tehran, a key backer of Syrian President Bashar al-Assad.

Writing in the New York Times, U.S. Secretary of State John Kerry urged "a united response led by the United States and the broadest possible coalition of nations” to combat ISIL.

U.S. President Barack Obama has acknowledged that Washington has no strategy yet to tackle ISIL, which has declared a "caliphate” in the territory under its control in Iraq and Syria.