The finding comes from an internal review of the incident, said the official, who spoke to The Associated Press on grounds of anonymity because the investigation is not complete.
"Errors were made" in the early May attack, the official acknowledged, discussing an incident that has strained relations between Washington and Kabul and caused resentment among the Afghan people.
American military commanders on a number of occasions have rewritten rules on U.S. bombing missions in an effort to avoid civilian casualties, most recently late last year. The official said these more restrictive rules were not followed in some of the air strikes May 4 in Afghanistan's western Farah province.
The report, which must still be circulated and briefed to other officials before it's final, was the most straightforward acknowledgment yet by the United States that mistakes were made in the strike. The story was first reported by The New York Times.
One example cited by the investigators, the official said, involved a U.S. warplane that got permission to launch an attack against a suspected Taliban site and proceeded toward carrying it out. For some reason, the plane circled back and didn't reconfirm the target before dropping the bomb. The official said that left the possibility that civilians had entered the area or that the Taliban had left.
President Hamid Karzai has urged more caution on the part of the allied forces, and he discussed the situation on a recent visit to Washington. Afghans say 140 civilians died, while American commanders say video evidence recorded by fighter jets and the account of the ground commander suggest no more than 30 civilians were killed, as well as 60-65 Taliban.
The International Committee of the Red Cross said a team that it sent to the area saw "dozens of bodies in each of the two locations," including women and children.
Army Lt. Gen. Stanley McChrystal, who has been chosen by the administration to replace Gen. David McKiernan as commander in Afghanistan, on Tuesday emphasized the importance of avoiding Afghan civilian casualties during his confirmation hearing before a Senate committee.
"This is a struggle for the support of the Afghan people. Our willingness to operate in ways that minimize casualties or damage - even when doing so makes our task more difficult - is essential to our credibility," McChrystal said. "I cannot overstate my commitment to the importance of this concept."
The general said he intends to review U.S. and allied operating procedures with an eye to minimizing civilian deaths. He also said that if he could obtain more intelligence, surveillance and reconnaissance aircraft, it would sharpen the precision of allied attacks, thereby avoiding unwanted casualties.
"I believe the perception caused by civilian casualties is one of the most dangerous things we face in Afghanistan, particularly with the Afghan people," he said. "We've got to recognize that that is a way to lose their faith and lose their support, and that would be strategically decisive against us."
Protecting the local population is a key tenet in counterinsurgency campaigns to "win hearts and minds" away from insurgents and for the nation's elected government. Karzai has repeatedly warned that civilian deaths are causing Afghans to side with the Taliban. The Obama administration is pouring more troops into the country to battle the raging insurgency.
According to the U.S. military, the battle in Farah province began a day after Taliban fighters entered the two villages, demanded money from civilians and killed three former government employees. An Afghan force rushed in, only to be ambushed by as many as 300 insurgents.
The provincial governor asked for U.S. military help, and American ground troops joined the battle, the U.S. says.
Before the battle was over, troops called in F-18 fighter jet airstrikes as well as help from a B-1 bomber, coordinating with the ground commander to strike a half dozen targets including buildings and a tree grove insurgents were firing from or massing in, the U.S. has said.
Officials have insisted they go to great lengths to avoid civilian casualties on the ground.
Maj. Gen. William Rew, the Air Force's director of operational planning, policy and strategy, said during a February meeting with reporters that "thousands" of attacks have been called off at the last minute when live video feeds from unmanned aerial vehicles showed civilians in the area of a planned strike.