SULAIMAN BEK, Iraq - Iraqi troops, Kurdish fighters and Shia militiamen backed by US air strikes pressed a fightback against militant-led militants Monday, buoyed by breaking a weeks-long siege of a town.

The military gains came as a senior UN rights official said the Islamic State (IS) militant group has carried out “acts of inhumanity on an unimaginable scale” in Iraq, while caretaker premier Nuri al-Maliki vowed the country would prove the group’s “graveyard”.

The breakthrough to the town of Amerli on Sunday was the biggest offensive success for the Iraqi government since IS-led militants overran much of the Sunni Arab heartland north and west of Baghdad in June.

The United States carried out limited air strikes in the area during the operation, the first time it has expanded its more than three-week air campaign against IS beyond the far north of Iraq.

Iraqi forces kept up the momentum on Monday, with Kurdish peshmerga fighters and Shia militiamen retaking Sulaiman Bek, a town north of Amerli that had been an important militant stronghold. “Within a few hours, we were able to clear the town completely,” the commander of the Shia Badr militia, Transport Minister Hadi al-Ameri, told AFP amid heavy security in Sulaiman Bek.

Fighters celebrated in the abandoned town, firing in the air, chanting slogans against IS and showing off a captured black flag of the group, an AFP journalist reported. Troops backed by Kurdish fighters and militiamen also surrounded the nearby town of Yankaja and pounded it with artillery and machine-gun fire as they fought to retake it from militants.

Before the operation, the mainly Shia Turkmen residents of Amerli were endangered both because of their faith, which militants consider heresy, and their resistance to the militants who besieged the town for 11 weeks.

Syrian troops and rebels battled close to the armistice line with Israel in the Golan Heights on Monday, as the UN pressed efforts to free 44 peacekeepers held by the insurgents.

The unrest on Israel’s doorstep followed a spillover of mortar and gunfire into Israeli-controlled territory, putting the Jewish state on high alert and prompting it to shoot down a drone over the occupied plateau on Sunday.

Several mortar rounds struck on the edges of the ceasefire line early Monday as the combatants exchanged rocket, mortar and tank fire near the Quneitra crossing, which Al-Qaeda-linked rebels seized last week, an AFP correspondent reported.

Human Rights Watch said Monday that Islamic State militants have used cluster munitions in Syria in at least one location and Syria’s regime is continuing to use the widely banned weapon.

The New York-based group, citing reports from local Kurdish officials and photographic evidence, said IS fighters had used cluster bombs on July 12 and August 14.

They were deployed in fighting around the town of Ayn al-Arab in Aleppo province, near the border with Turkey, in clashes between the militant group and local Kurdish fighters.

UN Iraq envoy Nickolay Mladenov had warned that they faced a “massacre” by the besieging militants. Maliki visited Amerli, 160 kilometres (100 miles) north of Baghdad on Monday, vowing that “Iraq will be a graveyard” for IS.

The operation to break the siege of the town was launched on Saturday after days of preparations.

The government’s reliance on Shia militiamen in this and other operations risks entrenching groups which themselves have a history of brutal sectarian killings.

The United States said it had carried out a total of four air strikes in the Amerli area, expanding its air campaign outside northern Iraq.

In doing so, it is supporting operations involving militia forces which previously fought against US troops in Iraq. David Petraeus, a former commander-in-chief of US-led forces in Iraq, has warned against America becoming an “air force for Shia militias”.

Meanwhile, more pledges were made to provide arms to Iraq’s Kurds, who are battling militants in northern and eastern Iraq.

Backed by US air strikes, Kurdish troops have succeeded in clawing back some areas of the north that they lost to the militants last month.

Germany will send anti-tank rocket launchers, rifles and hand grenades to support Kurdish forces, the German defence ministry announced.

Chancellor Angela Merkel made an impassioned 25-minute speech Monday in support of arming the Kurds, saying: “We have the opportunity to save lives and stop the further spread of mass murder in Iraq.”

She also said an estimated 400 German nationals had travelled to Iraq and Syria to fight alongside militants, and that “we must fear these fighters could return one day.”

The issue has raised fears in various Western countries, and British Prime Minister David Cameron was on Monday due to outline tougher measures against suspected militants.

The United Nations mission to Iraq said on Monday that violence in the country killed at least 1,420 people during August and wounded a further 1,370.

It said the figures did not include Anbar province, west of Baghdad, acknowledging difficulties in verifying information from conflict zones and areas outside government control.

IS and its allies control a large swathe of northeastern Syria as well as territory in Iraq, and its rule has been marked by atrocities that have shocked the world.

“The reports we have received reveal acts of inhumanity on an unimaginable scale,” deputy UN rights chief Flavia Pansieri said Monday.

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.