Smoke rises after a July 2017 airstrike by U.S.-led forces targeting the Islamic State in Mosul, Iraq. (Ahmad Al-Rubaye/AFP/Getty Images)

U.S. military operations killed approximately 500 civilians in 2017, as President Trump took command of the Pentagon and U.S. troops pursued the Islamic State and other militants overseas, a report made public on Friday found.

In a study mandated by Congress, the Defense Department said it considered allegations about those incidents, which took place in Afghanistan, Iraq, Syria and Yemen, to be credible after an internal review. Another 169 civilians were reported injured.

But watchdog groups and legal experts said the Pentagon may be grossly undercounting the number of noncombatants killed in airstrikes and other military activities because of a faulty system for investigating and counting possible deaths.

“The Defense Department has deemed that the vast majority of claims of civilian casualties are not credible without ever investigating them,” Daphne Eviatar, a director at Amnesty International USA, said in a statement. “Its numbers therefore likely severely undercount the actual civilian death toll.”

Airwars, an organization that tracks and examines casualty allegations using information from social media and other sources, reported this year that the United States and its allies may have killed as many as 6,000 civilians in strikes in Iraq and Syria alone in 2017.

In its study, required by the fiscal 2018 defense authorization law, the Pentagon said it had abided by laws of war, including those designed to protect civilians.

Defense officials have long defended the precautions they take to avoid accidentally hitting noncombatants, citing elaborate measures taken to surveil target sites and calculate blast areas before airstrikes. Military leaders including Defense Secretary Jim Mattis have called the U.S. operations against often-brutal militant groups highly precise.

But the challenge facing military leaders has increased as the fight is more often conducted from a distance, using aircraft and local partners to reduce harm to U.S. troops. That approach often reduces U.S. forces’ view of what is occurring in battlefields where militants and civilians exist in proximity.

The period covered by the report included intense U.S. air operations to expel the Islamic State from the Iraqi city of Mosul and the Syrian city of Raqqa; both offensives occurred in crowded urban areas.

The Pentagon said it and its allies conducted more than 10,000 strikes against the Islamic State in 2017, a massive air operation that presented a challenge to U.S. surveillance and intelligence capabilities.

The report also included an increasingly intense air war in Afghanistan, where U.S. commanders have been granted new authorities to target the Taliban and provide air support to Afghan troops.

Also in 2017, U.S. forces continued their counterterrorism campaign in Yemen, where a raid in January 2017 left a Navy SEAL dead and, according to Yemeni villagers, killed dozens of civilians.

The Pentagon study, which was first reported by CNN, found no credible allegations of civilian casualties in separate U.S. operations in Somalia and Libya.

Watchdog groups have long derided the Pentagon’s count as noncredible, accusing the department of failing to conduct interviews with witnesses and survivors. They also fault the military for a lack of widespread condolence payments.

In its study, the Pentagon acknowledged the divergent death tolls but said its analysis incorporated certain information unavailable to civilian investigators “such as operational planning and intelligence sources.”

Ryan Goodman, a professor at New York University School of Law and a former Pentagon official, argued in a recent op-ed that a central flaw in the military’s process is the high bar that must be cleared by those asserting harm before an investigation is opened: The military requires substantial indications that a death may have occurred. An even higher standard is required for confirming that a death took place.

Speaking Friday, Goodman said the report failed to include information requested by Congress about confirmed and “reasonably suspected” cases.

“I am sure Secretary Mattis as well as Congress want to know the true number of cases in which civilians may have been killed,” he said. “This report does not provide that number.”