Iraqi pro-government forces patrol the eastern part of Mosul on Dec. 28 during an ongoing military operation against the Islamic State. (Ahmad Al-Rubaye/AFP/Getty Images)

Iraqi forces launched a renewed assault on eastern neighborhoods of Mosul on Thursday, attempting to break a deadlock in the battle to retake the city from Islamic State militants.

The biggest military operation in Iraq since the 2003 U.S.-led invasion, the siege of Mosul is progressing more slowly than expected amid fierce resistance by the hardened militants of the extremist Islamic State group. The presence of more than a million civilians is also complicating the effort.

Before the operation for Mosul, Prime Minister Haider al-Abadi said he expected it to be retaken by year’s end, but only about a quarter of the city is back under government control. After two months of grinding fighting and heavy casualties, the Iraqi government paused the campaign earlier this month to give its forces a chance to regroup.

Adjusting his predictions, Abadi said this week that security forces will need three more months to defeat Islamic State militants in the country. Mosul is the group’s last major urban stronghold in Iraq, though it still controls other towns and villages.

Meanwhile, U.S. military officials said Thursday that during operations to liberate Mosul, an airstrike against militants in a hospital compound’s parking lot may have also inadvertently caused the deaths of an undisclosed number of civilians.

The strike targeted a van carrying Islamic State fighters who were fleeing after being seen firing a recoilless rifle, defense officials said. The U.S. military said that the incident will be “fully investigated” and that it “takes all allegations of civilian casualties seriously.”

The fresh three-pronged offensive increases pressure on the Islamic State’s ability to generate forces, move fighters or resupply, U.S. Central Command said in a statement. Units of the 16th Division of the Iraqi army pushed from the north, Iraq’s elite counterterrorism forces from the east and federal police from the south, it said.

“We are advancing very slowly to preserve the life of the civilians,” said Lt. Gen. Abdul Ghani al-Asadi, commander of counterterrorism forces, adding that his troops had already destroyed two car bombs. Thursday’s assault began at 7 a.m., and counterterrorism forces had entered the Karama and Quds neighborhoods, he said, securing part of the latter by the afternoon.

His force, known as the Golden Division, has borne the brunt of the fighting in Mosul after also leading assaults on other cities the militants had taken. Its highly trained fighters are hard to replace, raising questions about whether they can sustain the rate of losses they have suffered so far.

The Iraqi government does not release casualty figures and has restricted access to the front lines for journalists in recent weeks, but attrition rates have been high, according to soldiers and commanders.

Federal police units operating south of the city have also moved to neighborhoods in the east to reinforce army and counterterrorism units fighting there.

On Thursday, Brig. Gen. Walid Khalifa, deputy commander of the Iraqi army’s 9th Division, which is fighting in the city’s southeast, was sounding a more positive note. “We are facing a very limited resistance compared to the early days of the operation,” he said. “The enemy is collapsing, and it’s only a matter of days to control the whole eastern part.”

Mosul, the biggest city in Iraq’s north and jewel of the Islamic State’s self-proclaimed caliphate, is sliced in half by the Tigris River. Coalition airstrikes have knocked out all five bridges across the river, cutting the eastern side off from militant reinforcements.

Some commanders say the western side, with narrower streets and more civilians, will be an even tougher fight.

Army and police units progressing from the north and south of the city have made limited progress, and some of the gains announced Thursday appeared to be overblown.

Lt. Gen. Abdulameer Yarallah, commander of the Mosul operation, said forces in the north had retaken Tawela village, destroying eight car bombs. The Iraqi military had previously announced that it was retaken earlier this month.

The fall of Mosul in 2014 was the greatest victory for the Islamic State in Iraq, and soon afterward its leader declared from the city’s great mosque a new caliphate that would supposedly one day encompass the whole region.

Morris reported from Beirut. Paul Schemm in Addis Ababa, Ethiopia, and Dan Lamothe in Washington contributed to this report.