Two more girls were wounded in the explosion in eastern Nangarhar province, 100 miles east of the Afghan capital of Kabul, Xinhua News Agency reported.
"The innocent children were collecting firewood in Salozai area of Chaparhar district about 9 a.m., local time, when one of them stepped on a bomb triggering a powerful blast," provincial police spokesman Hazrat Housain Mashriqiwal told Xinhua.
He said those killed were 8 to 12 years old. Both the injured children, a boy and a girl, were in critical condition, the news agency said.
Police blamed Taliban insurgents for planting the explosive in the area to target security forces, Xinhua said.
Gen. John R. Allen, commander of the International Security Assistance Force, released a statement saying he is "deeply saddened by this news."
"Over three decades of conflict, Afghanistan became one of the most heavily mined countries on Earth. The tragic and cruel fact about landmines is that they don't discriminate. These precious children were innocent victims and I express my sincere sympathy and condolences to the families and friends of those affected."
There was no immediate of claim of responsibility for the incident, Xinhua said.