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Attackers have detonated explosives before storming the offices of the Save the Children charity in the eastern Afghan city of Jalalabad.
At least two people have been killed and 12 injured, officials say. It is believed about 50 staff were in the building at the time.
The Islamic State group has said three of its fighters are behind the attack, which is said to have now ended.
Save the Children has temporarily suspended all of its Afghan programmes.
The attack started at about 09:10 local time (04:40 GMT) on Wednesday when a suicide bomber detonated a vehicle explosive at the entrance to the Save the Children compound, Attaullah Khogyani, a spokesman for the governor of Nangarhar province, told the BBC.
About 45 people were reported to have been rescued from the basement, as fighting continued on the upper floors.
Earlier, one WhatsApp message, reported by AFP to be from an employee, had read: “I can hear two attackers… They are looking for us. Pray for us… Inform the security forces.”
There are several other aid agencies in the area, along with government offices.
The Islamic State group said in a message on its news outlet Amaq that three attackers and an explosives-laden car had targeted “British, Swedish and Afghan institutions in Jalalabad”.
The city has been a stronghold for IS, whose fighters have been active there since 2015.
The Taliban had earlier denied carrying out the attack.
Their fighters had been behind the storming of a luxury hotel in Kabul at the weekend that killed at least 22 people, mostly foreigners.
BBC