MOSUL, Iraq (AP) — Iraq’s prime minister on Tuesday declared eastern Mosul “fully liberated” from Islamic State group militants after a day of fierce fighting and more than three months after a massive U.S.-backed operation to retake the city began.

Iraqi forces drove Islamic State militants from one of their last bastions in the eastern half of the city, while aid groups expressed concern for the estimated 750,000 people still in the militant-held west.

In his weekly news conference on Tuesday, Haider al-Abadi hailed the “unmatched heroism of all security forces factions” and public support for the operation.

“Daesh has quickly collapsed and no one expected such collapse,” al-Abadi said, using the Arabia acronym of IS. “The heroism of our security forces was behind Daesh’s defeat.”

Mosul, Iraq’s second-largest city and the IS’s last urban stronghold in the country, fell into the hands of the extremists in the summer of 2014, when the group captured large swaths of northern and western Iraq.

Asked how long it will take to liberate the western side of the city, al-Abadi told The Associated Press: “I can’t tell now, but we are capable of doing so and we will do so.”

Hundreds of civilians fled from the northeastern Rashidiya neighborhood on foot as Iraqi helicopters circled overhead and fired on militants. At least two wounded Iraqi soldiers were brought back from the front lines after a suicide bombing.

A mortar attack in another neighborhood in eastern Mosul killed an Iraqi army colonel on Sunday, according to Brig. Gen. Yahya Rasool, a military spokesman.

Meanwhile, Al-Abadi renewed his promise to investigate allegations of human rights violations by security forces in conflict areas and bring those responsible to trial. His comments came a day after ordering a probe into a video on social media purportedly showing government troops beating and killing at least three IS suspects in Mosul.

On Monday, a provincial investigative committee in western Anbar province concluded its probe into human rights violations in June near the town of Fallujah. It found that a member of a Shiite militia killed 17 civilians, Al-Abadi said. The militiaman, affiliated with the Badr group, is now in detention and awaiting trial, he said.