Three staff members were killed and 20 people injured after gunmen stormed a regional office of Save The Children in Afghanistan's eastern Nangarhar province.
A police officer and a civilian also died during the attack in Jalalabad, according to Attahullah Khogyani, a spokesperson for the provincial governor.
Four of those wounded were Save the Children employees.
An insurgent carried out a suicide bombing and armed men entered the office and are said to have taken up positions on higher levels of the building.
Another insurgent was was killed by police and a third is said to still be fighting, sparking fears that the death toll could rise further.
So called Islamic State (IS) said it was behind the attack. They say they were targeting "British and Swedish foundations and Afghan government institutes".
In a statement, the charity condemned the attack in the "strongest possible terms".
"We are shocked and appalled at the violence, carried out against our staff in Afghanistan who are dedicated humanitarians, committed to improving the lives and wellbeing of millions of children across the country.
"We are doing everything we can to ensure all of our staff get the support they need in the aftermath of this devastating incident," the statement said.
Earlier, a spokesperson for Save the Children told ITV News: “"Our humanitarian work in Afghanistan reaches almost 1.4 million children. We remain committed to resuming our operations and lifesaving work as quickly as possible, as soon as we can be assured that it is safe to do so.”
Monica Zanarelli, the International Committee of the Red Cross' head of delegation in Afghanistan, branded the attack against an organisation that helps children "outrageous".
She said: "Civilians and aid workers must not be targeted. Increased violence has made operating in Afghanistan difficult for many organisations."
The attack on Save The Children's office come just days after a deadly siege of a hotel in Kabul left 22 people dead.
Among those killed during the 13-hour siege at the Intercontinental Hotel were 14 foreigners, including many Americans and seven Ukrainians.