Afghan security forces are seen on a street in Jalalabad. File picture: Rahman Safi/Xinhua

Kabul - The death toll from a terrorist attack on the aid organization Save the Children in Afghanistan has risen to six, the agency and an official confirmed on Thursday. 

Save the Children confirmed that four of its staff members were killed in the assault on their office in Jalalabad, the capital of the eastern province of Nangarhar.

A suicide bomber blew himself up outside the premises and militants stormed the building in a 10-hour-long assault on Wednesday claimed by the Islamic State terrorist group. 

"The identity of our late colleague was confirmed today, a young man in his 20s, during the second indepth sweep of the building," Save the Children said in a statement posted on Twitter, bringing the number of aid agency staff killed in the attack to four.

Two others, including an Afghan security force member and a civilian, were killed, Attaullah Khugyani, a spokesman for the provincial governor, told dpa. All five attackers were killed as well.

Twenty-five people, including four security guards and residents of nearby homes, were injured in the attack.

Afghan security forces rescued 46 people from the building as they battled the militants in the three-storey office, Khugyani said.

Save the Children said it has "temporarily suspended" its operations across Afghanistan. "The safety of our staff remains our primary concern."