"The US military takes precautions to minimise civilian loss of life during its operations - but obviously not enough," said John Sifton, Afghanistan researcher for Human Rights Watch on Saturday.
"There is now a pattern of mistakes, apparently as a result of faulty intelligence, that has led to too many civilian deaths and no clear changes in the way the United States plans and carries out military operations."
The American raids, which the US military said were aimed at "known terrorists", took place on 5 December and 6 December in Ghazni and Paktia provinces.
In the first, in a rural village near Gardez, six children and two adults died under a collapsed wall after a US air and ground attack on a compound used by a militant to store weapons.
The second, in a village in Ghazni province, A-10 "tankbuster" planes killed nine children, aged between nine and 12, and a young man. Both attacks failed to kill their intended targets.
Sifton said it was unclear why it had been necessary to use the A-10s' high-speed machineguns with exploding shells to attack a single person in a single house.
"The United States should reconsider the use of such weaponry in areas where there is clearly a considerable risk of civilian casualties," he said.
"The laws of war require that an attacking force take all feasible precautions to avoid civilian casualties. Even a cursory enquiry would have alerted US forces to the civilian presence in the area."