"This woman came to Afghanistan to teach Christianity to the people of Afghanistan," militant spokesman Zabiullah Mujahid told the Associated Press. "Our (leaders) issued a decree to kill this woman. This morning our people killed her in Kabul."
The gunmen drove by on a motorbike and shot to death the dual South African-British national, Interior Ministry spokesman Zemeri Bashary said, according to the AP.
The aid group Serving Emergency Relief and Vocational Enterprises, or SERVE Afghanistan, identified the woman as 34-year-old Gayle Williams. SERVE Afghanistan spokeswoman Rina van der Ende said none of the group's workers were proselytizing, which is prohibited by law in Afghanistan.
The group's Web site describes itself as a Christian charity registered in the UK that has been working with Afghan refugees since 1980 in Pakistan, and has gradually moved its work and its head office into Afghanistan.
"She was a person who always loved the Afghans and was dedicated to
serving those who are disabled," the organization said of Williams. "Needless to say we are all in shock."
Last year, a group of 23 South Korean aid workers from a church group were kidnapped in southern Afghanistan. Two were killed and the rest were released.
Monday's killing adds to a growing sense of insecurity in Afghanistan, according to the AP. Kabul is now dotted with police checkpoints. Embassies, military bases and the United Nations are erecting cement barriers to help protect against suicide bombers.
Meanwhile, NATO troops who were deployed last week to an insurgent stronghold west of Kabul in the Jalrez district in Wardak province engaged in two days of fighting and killed more than 20 militants, NATO reported Monday.
Militants have expanded their traditional bases in the south and east, along the border with Pakistan, to places closer to Kabul. Because of this, U.S. military officials have warned that the international mission to defeat the Taliban is in peril, and NATO generals have called for an increase in troops in Afghanistan.
NATO's Supreme Allied Commander Europe, U.S. Gen. John Craddock, also warned in a speech in London that NATO members are wavering in their political commitment to Afghanistan, Reuters reported.
"We are demonstrating a political will that is in my judgment sometimes wavering," he said. "It's this wavering political will that impedes operational progress and brings into question the relevance of the alliance here in the 21st century."
About 65,000 international troops, including about 32,000 Americans, are operating in Afghanistan, according to the AP.