MOSUL, Iraq — 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 ISIS. "The heroism of our security forces was behind Daesh's defeat."

Mosul, Iraq's second-largest city and ISIS'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 ISIS 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.

The U.N. Humanitarian Coordinator for Iraq, Lise Grande, expressed concern for civilians in the western half of Mosul in a statement signed by 20 international and local aid groups. She said the cost of food and basic goods is soaring, water and electricity are intermittent and that some residents are forced to burn furniture to keep warm.

"We hope that everything is done to protect the hundreds of thousands of people who are across the river in the west," Grande said in the statement. "We know that they are at extreme risk and we fear for their lives."

The statement called on warring parties "to uphold their obligations under international humanitarian law to protect civilians and ensure they have access to life-saving assistance."

Mosul is the group's last major urban bastion in Iraq. The extremists still control large areas in neighboring Syria.

In Geneva, a spokeswoman for the U.N. human rights office said it had received "reasonable corroboration" for a report that 19 civilians were killed in an airstrike in the al-Jadida neighborhood of IS-controlled western Mosul last week.

"Attributing responsibility for airstrikes is very difficult," Ravina Shamdasani said, adding that "it is clear that civilians are being killed in airstrikes."

A U.S.-led coalition and Iraq's own air force have been carrying out airstrikes in support of the Mosul operation.

The U.N. human rights office also said IS fighters have taken over "many hospitals" in western Mosul and are using them as military bases. It said the extremist group is diverting food, water and medicine to its fighters.

In Baghdad, a car exploded inside a dealership in the eastern Nahda area on Tueesday. The Interior Ministry said a bomb had been planted on the vehicle and that the blast caused no casualties. A police official and a medical official, speaking on condition of anonymity because they were not authorized to speak to reporters, said the blast killed at least two people and wounded seven.

Abdul-Zahra and Sinan Salaheddin reported from Baghdad. Associated Press writer Jamey Keaten in Geneva contributed to this report.