Iraqi forces broke through Sunday to the jihadist-besieged Shiite town of Amerli, 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 Sunni jihadist group Islamic State (IS) overran large areas of five provinces in June, sweeping security forces aside.

The breakthrough came as the United States carried out limited strikes in the area, the first time it has expanded its more than three-week air campaign against militants outside of Iraq's north.

Aircraft from several countries also dropped humanitarian aid to Amerli.

The mainly Shiite Turkmen residents of the town in Salaheddin province were running desperately short of food and water, and endangered both because of their Shiite faith, which jihadists consider heresy, 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.

A Peshmerga fighters gestures from an armed military vehicle as he guards a post in the strategic Jalawla area, in Diyala province, which is a gateway to Baghdad, as battles with Islamic State (IS) jihadists continue on August 25, 2014 Ali al-Saadi, AFP/File

"It is a very important success," Atta later said on state television.

Colonel Mustafa al-Bayati said Sunday night that the town of Amerli was "completely secure", but clashes were still ongoing in villages to its west.

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

Kurdish fighters and Shiite militiamen, meanwhile, clashed with militants who hold Sulaiman Bek and Yanakaja, north of Amerli.

The fighting killed two members of the Kurdish peshmerga forces, one of them a colonel, and 12 militiamen, an official responsible for the nearby Tuz Khurmatu area and a doctor said.

- US expands air campaign -

The government's reliance on the thousands of Shiite militiamen involved in the operation poses serious dangers for Iraq, risking entrenching groups with a history of brutal sectarian killings.

This US Air Forces Central Command handout photo shows a military loadmaster waiting the green light to release bundles of food and water for a humanitarian airdrop over the area of Amirli, Iraq on August 30, 2014 Staff Sgt. Shawn Nickel, US Air Forces Central Command/AFP

The United States announced that it carried out three air strikes in the Amerli area, expanding its air campaign outside northern Iraq, while Australian, British, French and US aircraft dropped relief supplies for the town.

"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 (IS) terrorists in order to support this humanitarian assistance operation", he added.

The American strikes were at least indirectly in support of an operation involving militia forces that previously fought against US troops in Iraq.

Three American air strikes near Amerli early on Sunday destroyed five IS vehicles and a checkpoint, bringing the total number of US strikes since August 8 to nearly 120.

US Vice President Joe Biden on Sunday spoke with Iraqi Kurdistan's regional president, Massud Barzani, on the humanitarian efforts and air strikes, the White House said.

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 force.

- Suicide bombings -

The US military also launched air strikes 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.

Two suicide bombers targeted security forces on Sunday in Ramadi, a city west of Baghdad where Iraqi forces have struggled to regain control areas from militants.

Smoke rises in the horizon following US airstrikes targeting Islamic State (IS) militants at Mosul Dam on the outskirts of the northern city of Mosul where insurgents are fighting Kurdish forces on August 18, 2014 Ahmad al-Rubaye, AFP/File

The blasts killed 13 people and wounded 17, police and a doctor said.

The IS and its allies control significant areas north and west of Baghdad and in neighbouring northeastern Syria.

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 IS, but has so far ruled out any cooperation with the Damascus regime against the jihadists.

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

US Secretary of State John Kerry has urged "a united response led by the United States and the broadest possible coalition of nations" to combat IS.