U.S. military operations killed approximately 500 civilians in 2017, as President Donald 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 new 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 due to 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, an official at Amnesty International USA, said in a statement. "Its numbers therefore likely severely undercount the actual civilian death toll."

Airwars, which tracks and examines casualty allegations using social media and other information, reported earlier 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 accidental targeting of 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. But the approach often reduces U.S. forces' visibility 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 of which occurred in a crowded urban context.

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 deadly 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 civilian casualty allegations in separate U.S. operations in Somalia and Libya.

Watchdog groups have long derided the Pentagon's count as noncredible, criticizing the department for failing to conduct interviews with witnesses and survivors. They also fault the military for its 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 Law School and former Pentagon official, argued in a recent op-ed piece that a central flaw to the military process was the high bar, requiring substantial indications that a death may have occurred, set for opening an investigation. An even higher standard is required for confirming 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."