BAGHDAD- Iraqi forces broke through to the militant-besieged 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 Islamic State (IS) militant group overran large areas of five provinces in June, sweeping security forces aside. The breakthrough came as America 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 Turkmen residents of the town in Salaheddin province were running desperately short of food and water, and endangered both because of their faith, which militants 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. "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, militiamen and Kurdish fighters deployed for the assault and Iraqi aircraft carried out strikes against militants.

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

The US announced that it carried out three air strikes in the Amerli area, expanding its air campaign outside the far north for the first time, 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 US 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 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. "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.

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

Meanwhile, thirty seven people were killed when a suicide bomber rammed an explosives-packed Humvee military vehicle into a construction site, used by the army and police, according to two police officers and a medical official. The blast killed 22 security personnel and 15 civilians, the medical and police sources said. The nine-storey building under construction, used as a base by security personnel, was located in the centre of Ramadi in western Anbar province. Ramadi and its sister city Falluja have been plagued by fighting between the security forces and the extremist Islamic State and other armed groups since January.

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 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 govt of Iraq, the UN, and non-govt organisations to provide humanitarian assistance in Iraq as needed," Kirby said.

The US military also launched 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 militants earlier this month, securing the source of much of the power and irrigation water for the region around Iraq's second city.

The militant 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 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 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, 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.

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.