The international relief agency Medecins Sans Frontieres has suspended operations in Afghanistan after five of its workers were killed in an ambush.
Aid agencies are concerned about deteriorating security
The country's former Taleban rulers say they carried out Wednesday's attack in the north-west of the country.
Correspondents say it was one of the deadliest since the radical Islamic militia was ousted in late 2001.
An MSF Spokeswoman said the priority now was to take care of people who had been most affected by the killings.
The attack is reported to have been carried out carried by attackers on a motorcycle who shot at a four-wheel-drive MSF vehicle with assault rifles and grenades in in Badghis province, 550 km (340 miles) west of Kabul.
Two aid workers - one Dutch and one Belgian - a Norwegian doctor and their Afghan driver and translator were killed.
"For the time being, our activities will be suspended nationwide," MSF spokeswoman Vicky Hawkins told a news conference.
"In the coming weeks we will analyse this event in-depth, but for the moment our priority is to take care of those most affected by this tragedy," she said.
MSF employs 80 expatriates and 1,400 local people in its Afghanistan operations.
Correspondents say the suspension reflects fears among relief agencies that a Taleban insurgency in the south and east of the country could be spreading and severely limit their operations.
"Today our thoughts are with those of the families of those killed but also with the people of Afghanistan whose ability to access healthcare and other humanitarian assistance is becoming increasingly compromised," said Ms Hawkins.
The BBC's Charles Haviland in Kabul says that attacks on foreigners have increased recently, and at least 13 aid workers have been killed in the country this year.