KABUL, Afghanistan — Child soldiers recruited by the Taliban were used in the battle to overrun the northern city of Kunduz last year, a prominent human rights organization said on Wednesday.

Human Rights Watch reported 13 cases of children recruited to fight for the Taliban in that battle. Its researchers found that the insurgents used Islamic religious schools in the area “to provide military training to children between the ages of 13 and 17, many of whom have been deployed in combat.”

Human Rights Watch said the Taliban begin indoctrinating children from as early as 6 years old. The United Nations has documented that children as young as 10 participated in the fighting in Kunduz, the capital of Kunduz Province.

That northern city was captured in September by Taliban fighters; government forces, backed by United States air power, reclaimed the city days later.