Airstrikes destroyed around 200 vehicles believed to be carrying ISIS fighters fleeing one of their former strongholds, a senior Iraqi official told NBC News on Thursday.
The attack targeting a convoy killed an unknown number of people.
Brig. Yahya Rasool, a spokesman for Iraq's Joint Operation Command, said helicopters and warplanes fired at vehicles on road about 12 miles south of Fallujah on Wednesday.
"All ISIS militants traveling in these vehicles were killed," he said without providing a number of people who died.
Although the Iraqi military initially said that it had acted alone, the U.S. Embassy in Baghdad later confirmed the U.S. had participated in the strikes.
"We started to chase ISIS convoys on Tuesday. On the same day, the U.S.-led coalition destroyed those convoys alongside Iraqi F-16s," the embassy tweeted in Arabic on Thursday.
A senior American defense official said U.S. strikes had "hit a large number of vehicles."
Speaking on condition of anonymity as they were not authorized to speak to the press, the official said initial U.S. estimates suggested that aircraft involved in the operation may have killed as many as 240 people in more than 50 vehicles. However, the official cautioned that those numbers could change.
Reuters quoted an unidentified U.S. official as saying a preliminary estimate suggested at least 250 suspected fighters had been killed.
It was unclear how the convoy was identified as being made up entirely of vehicles belonging to ISIS militants. Civilians are known to have been displaced in the area where the attack occurred.
The discrepancies in numbers and details offered by the U.S. and Iraq could not immediately be reconciled. NBC News was unable independently confirm the different accounts.
The alleged fighters were trying to escape the previously ISIS-held Fallujah area, Iraqi officials said.
A senior Iraqi commander declared Sunday that Fallujah was "fully liberated" from ISIS militants after a more than month-long military operation.
The Fallujah liberation has fueled an exodus of thousands of families, overwhelming camps for the displaced run by the government and aid groups.
Sadiq Taher told NBC News he witnessed part of the attack on the suspected ISIS convoy. The 25-year-old volunteer aid worker had bedded down for the night on the highway with other members of his team after their food-laden trucks got stuck in a rut south of Fallujah.
It was still dark early Wednesday when gunfire and explosions in the distance woke up the group. Soon, a convoy of vehicles traveling at speed with their lights off approached the stranded trucks, Taher said.
The cars were carrying militants and some were equipped with mounted machine guns, according to Taher. Terrified, the three men abandoned the aid trucks and ran into the desert to hide, stripping to their underwear and covering themselves in dirt. A few of the fighters stopped to investigate — some were 14 feet from the trio — but eventually drove on, Taher said.
Within minutes, the hiding men heard helicopters overhead and saw flashes as airstrikes hit the alleged ISIS vehicles.
"I heard the sounds of the explosions and I saw the light of the burning vehicles," Taher said.
He recalled their dilemma — if they ran, they feared the Iraqi helicopters would also attack them.
"I was afraid of being discovered by ISIS militants or to be targeted by Iraqi military helicopters," he said. "But I had to think wisely, calmly and keep silent."
As dawn broke, Taher could still see militants abandoning their cars and fleeing by foot into the desert.
The airstrikes over, the men waited until the afternoon and then returned to Baghdad.
ISIS did not confiscate the food aid, according to Jeremy Courtney, the co-founder and executive director of aid group Preemptive Love Coalition.
"Our team is determined to still deliver this much-needed support to women and children the elderly and families who have been waiting for its arrival for days," he told NBC News from Baghdad.
In a separate incident on Wednesday, Iraqi helicopter strikes killed some 150 alleged ISIS militants as they tried to reach neighboring Syria, Rasool said.
"They were trying to escape in order to reach Syria then the Iraqi helicopters found a place ISIS were hiding," he added.
That attack occurred around 2 miles west of Amiriyah Fallujah, according to Rasool.