Police said they believed that the device was an unexploded mine that had been laid years ago and was triggered somehow as the girls walked through the open field.
At least one other old mine was found nearby, provincial police spokesman Hazrat Hussian Mashreqiwal said. He also noted that the blast did not occur next to a road or any obvious target.
The girls who died ranged in age from 9 to 13 years old and all came from different families in Dawlatzai village, said Mohammad Seddiq, the government administrator Nangarhar province's Chaperhar district, which includes the vilage.
Two more girls were seriously wounded and are in critical condition at a hospital, Seddiq said. He spoke to The Associated Press by phone from the site of the blast.
Afghanistan remains one of the most heavily landmined countries in the world despite years of clearance.
Many mines are left in rural areas from the 1990s and discovered only when they are triggered accidentally.
(Copyright 2012 by The Associated Press. All Rights Reserved.)