Najibullah Kamawal, director of the health department in the eastern Nangarhar province, says another 65 people were wounded in Saturday's attack.
The bomber targeted a gathering of fighters who were celebrating a three-day truce coinciding with the Eid al-Fitr holiday.
No one immediately claimed the attack, but it was likely carried out by Daesh (also known as ISIL or ISIS), which was not included in the cease-fire and has clashed with the Taliban in the past, AP reported.
Later the group said in its Amaq website said the attack on Saturday targeted "a gathering of Afghan forces" in Nangarhar, but gave no details.
After the attack, Afghan President Ashraf Ghani announced a nine-day extension of the cease-fire. T