Attackers have detonated explosives before storming the offices of the Save the Children charity in the eastern Afghan city of Jalalabad.
At least 11 people are reported injured so far. It is believed about 50 staff were in the building at the time.
A suspected suicide car bomb blast began the assault, with the gunmen now said to be using machine guns and RPGs from higher floors in the building.
No group has yet said it is behind Wednesday's attack.
What's the latest on the attack?
It started at about 09:10 local time (04:40 GMT) when a suicide attacker detonated a car bomb at the entrance to the Save the Children compound, Ataullah Khogyani, a provincial government spokesperson, told the BBC.
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 (RPG).
Local journalist Bilal Sarwary said police had told him Afghan commandos were at the scene and that the attackers were on the upper floors of the building, using heavy machine guns, grenades and RPGs.
He says initial reports suggest there are two or three attackers. He also says a doctor has told him at least one Afghan soldier has been killed.
One WhatsApp message, reported by AFP to be from an employee, 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.
Who is behind it?
Uncertain as yet.
Jalalabad, near the border with Pakistan, is often targeted by Taliban militants but it is also a stronghold for the Islamic State group, whose fighters have been active there since 2015.
The latest attack comes days after Taliban gunmen stormed a luxury hotel in Kabul killing at least 22 people, mostly foreigners.
But in a Twitter message the group denied carrying out the Jalalabad attack.
What has Save the Children said?
An emailed statement from a Save the Children spokesperson said the group was "devastated" at the news of the attack, adding: "Our primary concern is for the safety and security of our staff."
It added: "We are awaiting further information from our team and cannot comment further at this time."
The UN's mission in Afghanistan said: "Attacks directed at civilians or aid organisations are clear violations of international humanitarian law and may amount to war crimes."
What is Save the Children's Afghan work?
The charity has been working in Afghanistan since 1976. It currently runs programmes across 16 provinces in Afghanistan.
According to the aid agency, more than 700,000 children in Afghanistan have been reached over the years through its efforts.
The organisation says it aims to provide better access to education, healthcare and essential supplies to children across the globe.
Are charity groups targeted in Afghanistan?
They continue to work under tough conditions in the country, facing regular attacks and kidnappings.
The Red Cross announced in October that it was drastically reducing its presence in Afghanistan after seven of its staff were killed in attacks in 2017.
Attacks over the years include:
May 2017: Attackers storm a guesthouse run by a Swedish NGO, Operation Mercy, killing a German woman and Afghan guard
July 2014: Gunmen shoot dead two Finnish women working for a Christian aid charity, the International Assistance Mission (IAM), in the western city of Herat
October 2010: Kidnapped UK aid worker Linda Norgrove is killed in a rescue attempt
August 2010: 10 members of an eye care team working for IAM are shot dead in Nuristan province
Additionally, the US bombing of a Médecins Sans Frontières hospital in Kunduz in October 2015 killed 22 people.
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