Ciaran Donnelly, director of operations in Afghanistan for the International Rescue Committee, said the entire aid community has been affected by worsening security.
"We're not the only NGO (nongovernment organization) to have suffered an attack. Unfortunately we suffered the most egregious and most tragic of these attacks," he said Saturday.
Taliban fighters wielding Kalashnikov assault rifles killed four IRC workers, including three women, in an attack Wednesday in Logar province, just south of Kabul. A Trinidadian-American, a British-Canadian and a Canadian were killed along with their Afghan driver.
The ambush of two clearly marked aid vehicles on the main road south of Kabul was the latest in a record number of attacks on aid groups this year - a surge that has workers questioning if they can safely provide services in remote and dangerous areas where help is most needed.
The Taliban claimed responsibility and said the women were linked to a Western military.
Despite the increasing danger, no aid groups have yet pulled out of Afghanistan, though some groups might suspend projects or move personnel out of dangerous regions, said Mohammad Hashim Mayar, deputy director for the Agency Coordinating Body for Afghan Relief, an umbrella group for aid organizations in Afghanistan.
The president of the IRC, George Rupp, said Saturday that it was the worst tragedy in the group's 75-year history. The IRC has 10 international staff and 500 Afghan staff in Afghanistan. It has temporarily suspended operations.
"We are not only deeply saddened but also outraged at this unprovoked and wanton slaughter of four innocent victims who were committed to providing humanitarian assistance to the Afghan people on an impartial basis of only meeting the needs of the vulnerable," Rupp told a news conference in Kabul.
Donnelly said the IRC hadn't yet determined if their vehicle was targeted because militants knew it belonged to IRC or simply because it carried Westerners. He said the group hadn't received any threats or warnings before the attack.
The IRC, which has operated in Afghanistan since 1988, carries out educational programs and helps refugees.
The two IRC officials said they were concerned that international militaries are taking on humanitarian projects, potentially blurring the lines in the eyes of locals or militants between what humanitarian groups do and what the military does.
NATO militaries carry out reconstruction projects throughout the country.
"There are very real ethical and operational concerns that arise from the confusion between humanitarian and political and military objectives," Donnelly said.
Attacks against aid workers in Afghanistan have spiked this year. Wednesday's assault brings the number of aid workers killed in militant attacks to at least 23 compared with 15 killed in all of 2007, according to a recent report from ANSO, a security group that works for aid organizations in the country.
ANSO said 2008 was on track to be the deadliest year for aid workers in Afghanistan since the 2001 U.S.-led invasion.