Brothers Taha, left, and Hasul are treated at a field hospital after being injured by a car bomb explosion during fighting between Iraqi forces and Islamic State militants in eastern Mosul, Iraq, on Nov. 25. (Felipe Dana/Associated Press)

Air and artillery strikes by the U.S.-led coalition in the Iraqi city of Mosul appear to have killed more than 80 civilians since a major operation there began last month, a watchdog group said Friday.

Airwars, a Britain-based group that tracks information from activists and other sources, said that the frequency of casualty reports had increased since Iraqi troops launched their offensive to clear the city of Islamic State militants on Oct. 17.

In an email, Airwars director Chris Woods said the group believed that coalition strikes, including air and artillery attacks, since then had likely killed 84 to 87 people in Mosul and injured more than 160 others.

The densely packed urban battlefield makes accurately determining deaths tolls extremely difficult, Woods said. The wide array of armed groups involved in the operation, moreover, makes it hard to establish who is responsible for any given attack.

Airwars draws its estimates from a host of sources, including activists, local media organizations and its own research. The U.S. military conducts its own investigations of civilian casualty reports, sometimes confirming the allegations and sometimes finding they are unjustified.

While some Airwars findings have been challenged by the U.S. military, its investigations have prompted U.S. Central Command, which oversees Middle East operations, to increase its estimate of the number of civilians killed in U.S. strikes.

Centcom says that American strikes against the Islamic State have killed at least 119 civilians in Iraq and Syria since they began in 2014, an estimate that some advocacy groups say is too low.

Col. John Thomas, a spokesman for Centcom, said the U.S. military conducts extensive checks before launching airstrikes, using intelligence and surveillance in an attempt to verify targets. He said U.S. and allied strikes had destroyed dozens of car bombs and tunnels that militants have used to attack encroaching forces.

But the challenge of avoiding inadvertent deaths has increased in such a large city, where more than a million people lived under two years of militant rule.

“The liberation of Mosul is an operation that is an order of magnitude larger and more complex than any of the previous cities that have been liberated from ISIL,” Thomas said in an email, using another acronym for the Islamic State.

Civilian deaths have become a major problem since Iraqi forces began to press deeper into Mosul in recent weeks. Islamic State militants have blocked some civilians from fleeing, while others have been caught in the crossfire. More recently, the fighters have been shelling recaptured areas, putting civilians in danger even after extremists are cleared from their neighborhoods.

The intense urban battle has challenged even Iraq’s most seasoned commandos and has cost its military significant losses.

Woods said that the vast majority of civilian deaths in the Mosul operation have been caused by the Islamic State.

U.S.-backed forces are also marching on the Syrian city of Raqqa, another important Islamic State stronghold, ahead of a battle that could face challenges similar to those in Mosul.