The Iraqi army began an operation on Monday to storm Falluja, the Islamic State group’s stronghold near Baghdad, a military officer told a Reuters TV crew near the front line.

A military unit is trying to advance in the city, he said, as explosions and gunfire could be heard in Falluja’s southern Naimiya district.

The Iraqi army, supported by Iranian-backed Shi’ite militia, began the operation to recapture Falluja on May 23 by first tightening its siege around the city, 50 km (30 miles) west of Baghdad.

In January 2014, Falluja became the first Iraqi city to fall under the control of the militants, six months before they declared a caliphate over territory seized in Iraq and Syria