The United Nations has said it regrets the decision of the aid agency Medecins Sans Frontieres to pull out of Afghanistan because of staff killings.
MSF says it may return when security improves
A UN spokesman in Kabul said that while the reasons were understood, MSF would be missed.
MSF officials said on Thursday it could return if security improved.
A spokesperson for the agency that co-ordinates Afghan relief said other charities were not planning to withdraw from the country.
"The United Nations Assistance Mission in Afghanistan and the UN system in general in Afghanistan learned with regret of the decision by MSF to leave," said UN spokesman Manoel de Almeida e Silva.
MEDECINS SANS FRONTIERES
Founded in 1971 by group of French doctors
Headquarters in Brussels and offices in 18 countries
Has operations in more than 80 countries
Has more than 2,000 expatriate volunteers, with an average age of 30-35
Work includes restoring hospitals, providing vaccinations and improving sanitation and water supply
In Afghanistan, continued to work through Soviet invasion, mujahideen wars and Taleban rule
"We can appreciate their reasons," he said, "and they will be missed... security is indeed a matter of concern."
Mr Silva appealed for aid agencies remaining in Afghanistan to maintain a dialogue with the US military after MSF's complaints that American forces were using humanitarian assistance to advance their strategic interests.
The US military in Afghanistan denied the allegation.
Anja Debeer, executive coordinator for the Agency Coordinating Body for Afghan Relief, said other aid groups were not planning to quit.
"At the moment nobody is considering withdrawing. It would be highly inappropriate to withdraw."
However, the agency believes increasing violence has badly hurt aid efforts and many groups have scaled back operations.
The UN also expressed its "outrage" over Wednesday's killing of two people in a bomb blast at a voter registration site in the central Afghan province of Ghazni.
It described the attack, which the Taleban is reported to have said it carried out, as "callous".
MSF meanwhile closed its head office in Kabul on Thursday after announcing it was pulling out because of the killing of five of its staff and the risk of further attacks.
Three European and two Afghan staff died in the attack on MSF in north-west Afghanistan in June. A Taleban spokesman said it carried out the attack.
MSF said it was unhappy with a government probe into the deaths and was leaving after 24 years in the country.
But officials said that rent would still be paid on the head office so that it would be possible at some stage to come back.
Afghan President Hamid Karzai said on Thursday that the government was "fully committed" to investigating the killings, but an MSF spokesman said his assurances were not enough to prevent it from leaving.