But a military spokesman, Lieutenant-Colonel Brayan Hilferty, said the report into the 6 December incident in the province of Ghazni remained "top secret", despite United Nations calls for it to be made public.
"The investigating officer said we used appropriate rules of engagement and did follow the law of conflict ...
However we did slightly change our rules of engagement after that investigation."
The US military used A-10 "tankbuster" planes in the attack on the compound of a suspected fighter in Ghazni province. They killed nine children, aged between nine and 12, and a young man.
The previous day in the neighbouring province of Gardez, six children and two adults were found dead under a collapsed wall after a US air and ground attack on another compound, which allegedly was used to store weapons.
Both attacks failed to kill their intended targets.
Hilferty did not refer to the Gardez incident.
He said the law of land warfare was agreed by most nations.
"It lays out certain rules and procedures to be followed, such as the law of proportionality, which is if you fire at me from that house across the street with a rifle, I should not reply with tens of bombs and destroy the entire block."
"You can follow all of the laws of land warfare and still unfortunately have tragic incidents," he added.
Rights group critical
Shortly after Ghazni attack, New York-based Human Rights Watch questioned why it had been necessary to use high-speed machine guns firing explosive rounds to target an individual.
"The investigating officer said we used appropriate rules of engagement and did follow the law of conflict ... however we did slightly change our rules of engagement after that investigation"
Lieutenant-Colonel Bryan Hilferty,
The United Nations called at the time for the results of the investigation to be made public.
Hilferty said they remained classified "because of the intelligence involved and the target involved". He declined to say how the rules of engagement had been altered.
"We don't discuss specifics of the rules of engagement in order not to give an advantage to the enemy," he said.
The United States has been criticised by many Afghans and rights groups for inflicting civilian casualties in its war in pursuit of Taliban, al-Qaida and allied fighters launched after the 11 September 2001 attacks on New York and Washington.
In a report released on Monday, Human Rights watch accused the United States of using excessive force when arresting suspected fighters, resulting in avoidable civilian deaths and possibly violating international law.
It particularly criticised "suppressing", or indiscriminate fire used in arrest operations to immobilise possible enemy forces.