IRBIL, Iraq -- Iraqi forces on Sunday began penetrating the narrow streets and warrens of Mosul's heavily populated Old City in the last phase of a monthslong operation against Islamic State militants, a battle that U.S. commanders have described as one of the toughest in urban warfare since World War II.
The assault began at dawn, with airstrikes and a push by Iraq's counterterrorism forces into the neighborhoods of the Old City. They were met with fierce resistance by Islamic State fighters, according to commanders, suggesting that the assault, the most vicious phase in the long fight for Mosul, could go on for days or weeks.
Iraqi special forces, the army and Federal Police are taking part in the operation to recapture the Old City, said Lt. Gen. Abdul-Amir Rasheed Yar Allah, who commands army operations in Ninevah province.
As the fight began, security forces broadcast a message to residents of the Old City over loudspeakers: "The security forces are advancing toward you and the salvation hour is near, and the enemy is losing its positions one after another, and their end is near."
The message continued, asking civilians to flee toward security forces: "Come toward your brothers, your armed forces, and you will find the proper care."
The area of the Old City sits on the western bank of the Tigris River and is home to the Great Mosque of al-Nuri, whose distinctive leaning minaret dominates western Mosul's skyline and where the Islamic State leader, Abu Bakr al-Baghdadi, in 2014 spoke from the pulpit and declared his caliphate.
The battle for Mosul -- the largest city the Islamic State has controlled in a vast territory straddling the border between Iraq and Syria -- is already in its ninth month.
The campaign has proved to be longer and tougher than anyone anticipated, with hundreds of casualties among Iraqi security forces and civilians, many caused by U.S.-led airstrikes in the densely populated city.
On Sunday morning there were reports, which could not be immediately confirmed, that more than a dozen civilians had been killed in airstrikes on the Old City.
The battle has tested humanitarian aid groups and added to the crisis that has engulfed Iraq since 2014, when the Islamic State captured Mosul and swept across the north, west and center of the country. Since then, 4.8 million Iraqis have fled their homes.
About 3 million have been unable to return home, according to the United Nations.
Humanitarian groups have been warning for weeks about the perils for civilians in the Old City, where the U.N. believes up to 150,000 people are trapped, running low on food and water and held by Islamic State fighters as human shields in the face of advancing security forces.
Since the Mosul battle began in October, more than 800,000 civilians have fled the fighting.
Some residents have since returned home, especially those from eastern Mosul, which Iraqi forces liberated earlier this year. But close to 700,000 civilians from the city's west have fled since February, and they remain displaced.
The International Rescue Committee called on Iraqi forces and the U.S.-led coalition to "do everything in their power to keep civilians safe during these final stages of the battle for Mosul."
"With its narrow and winding streets, Iraqi forces will be even more reliant on airstrikes despite the difficulty in identifying civilians sheltering in buildings and the increased risk of civilians being used as human shields by ISIS fighters," said Nora Love, the aid group's acting director for Iraq, using an acronym for the Islamic State.
Those who try to flee to government-controlled areas risk being caught in the crossfire or targeted by Islamic State snipers, Love added.
Gen. Abdel Ghani al-Asadi, the head of Iraq's special forces, told state TV that he expects the extremists to put up a "vicious and tough fight." Al-Asadi said the troops "will be very careful" to protect the civilians.
Information for this article was contributed by Tim Arango and Falih Hassan of The New York Times and by Sinan Salaheddin of The Associated Press.
A Section on 06/19/2017