BAGHDAD (AP) — A senior Iranian Revolutionary Guard commander was killed in an explosion during clashes with the Islamic State group west of Mosul, an Iraqi official told The Associated Press on Saturday, as aid groups voiced concern for the safety of civilians after Iraq's government called for residents in militant-held neighborhoods to flee immediately.

Gen. Shaaban Nasiiri was an adviser to Qassem Soleimani, the head of Iran's elite Quds Force. Soleimani has acted as a key adviser to Iraq's Popular Mobilization Forces — an umbrella group of Mostly Shiite militia forces sanctioned by the Iraqi government — in the fight against IS since 2014.

The Iraqi official, speaking on condition of anonymity in line with regulations, said Nasiiri was killed Friday and is the first senior Iranian commander to die in the Mosul fight.

Inside Mosul, U.S.-backed Iraqi forces began the push to retake the Old City on Saturday morning, moving in on the district from three directions, according to a statement from Nineveh operations command, the authority overseeing the Mosul fight.

The IS hold on Mosul has shrunk to just a handful of neighborhoods in and around the Old City district where narrow streets and a dense civilian population is expected to complicate the fight there.

Iraqi planes dropped leaflets over the area Friday telling civilians to flee "immediately" to "safe passages" where they will be greeted by "guides, protectors and (transportation) to reach safe places," according to a government statement.

However, it is unclear how the government intends to ensure safe passage for civilians as IS fighters have repeatedly targeted fleeing civilians with small arms and mortar fire.

The move to clear the Old City marks a shift in approach. Since the Mosul operation was launched in October, Iraqi forces have encouraged civilians to remain in their homes to avoid massive displacement. However, more than 730,000 people have fled the fight to date according to United Nations figures.

"As many as 200,000 additional people may try to leave in coming days," the U.N. said Saturday in a statement following the call for Old City civilians to leave. Save the Children warned that fleeing civilians could be caught in the crossfire, leading to "deadly chaos" in a statement Friday.

Both Iraqi forces and IS fighters are obligated under international law to protect civilians, the U.N. statement added.

More than 100,000 civilians are estimated to still be inside IS-held Mosul neighborhoods.

While U.S.-backed forces have fought inside Mosul during the operation to retake it from IS, Iraq's Popular Mobilization Forces have largely operated in the deserts to the west cutting supply lines and attempting to begin securing Iraq's border with Syria.

The Popular Mobilization Forces are largely supported by Tehran, a key Iraqi ally in the fight against IS. Iran has provided weapons, training and advisers credited with important early victories against the extremists in 2014 before the U.S. began a campaign of airstrikes targeting the group.

Mosul's eastern half was declared liberated in January and the push for the city's west began the following month. While some Iraqi commanders said they hoped to retake the city before Ramadan, the Muslim holy month that began Friday night, grueling urban combat has repeatedly slowed the pace of operations.