Attackers have detonated explosives before storming the offices of the save the children charity in the eastern Afghan city of Jalalabad.
Officials says at least two people have been killed and 12 injured. it is believed about 50 staff were in the Building at the time.
At least three gunmen are involved in the attack, reports say, with fighting continuing on the upper floor.
No group has yet said it was behind the attack.
A spokesman for the governor of Nangarhar province, told the BBC the attack started at about 09:10 local time when a suicide bomber detonated a vehicle explosive at the entrance to the save the children compound.
An eyewitness who was inside the compound at the time told afp news agency that he saw a gunman hitting the main gate with a rocket-propelled grenade.
Unconfirmed reports say at least two attackers have also been killed.