The Taliban claimed responsibility.
"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 aid group SERVE - Serving Emergency Relief and Vocational Enterprises - identified the woman as 34-year-old Gayle Williams. A spokeswoman for the group in Kabul denied that its workers were proselytizing, which is prohibited by law in Afghanistan.
"It's not the case that they preach, not at all," said the spokeswoman, Rina Vamberende.
In a statement on its Web site, SERVE described Williams as "a person who always loved the Afghans and was dedicated to serving those who are disabled."
The group describes itself as a Christian charity registered in Britain. The Web site says it has been working with Afghan refugees since 1980 in Pakistan.
"SERVE Afghanistan's purpose is to express God's love and bring hope by serving the people of Afghanistan, especially the needy, as we seek to address personal, social and environmental needs," the site says.
Last year a group of 23 South Korean aid workers from a church group were taken hostage in southern Afghanistan. Two were killed and the rest were released.
In 2001, eight international aid workers, including two Americans, were imprisoned and charged with preaching Christianity. The eight were freed by Afghan mujahedeen fighters attacking the Taliban after the U.S.-led invasion.
Monday's attack adds to a growing sense of insecurity in Kabul. The capital city is now blanketed with police checkpoints. Embassies, military bases and the U.N. are erecting cement barriers to guard against suicide bombings.
Kidnappings targeting wealthy Afghans have long been a problem in Kabul, but attacks against Westerners in the city and surrounding provinces have also increased recently. In mid-August, Taliban militants killed three women working for the U.S. aid group International Rescue Committee while they were driving in Logar, one province south of Kabul.
To the west of Kabul, assault helicopters dropped NATO troops into Jalrez district in Wardak province on Thursday, sparking a two-day battle involving airstrikes, the military alliance said in a statement Monday.
More than 20 militants were killed.
Wardak province, just 40 miles west of Kabul, has become an insurgent stronghold.
Militants have expanded their traditional bases in the country's south and east - along the border with Pakistan - and have gained territory in the provinces surrounding Kabul, a worrying development for Afghan and NATO troops.
Those advances are part of the reason that top U.S. military officials have warned that the international mission to defeat the Taliban is in peril, and why NATO generals have called for a sharp increase in the number of troops here.
Some 65,000 international troops now operate in Afghanistan, including about 32,000 Americans.
In northern Afghanistan, a suicide bomber killed two German soldiers and five children in Kunduz province, said Mohammad Omar, the provincial governor. NATO confirmed that some of its soldiers were killed and wounded in the attack.
Omar said the soldiers were patrolling on foot when the bomber riding a bicycle hit them. Northern Afghanistan has been spared much of the violence afflicting Afghanistan's eastern and southern provinces.
In the south, an operation Sunday evening by international and Afghan forces killed 34 Taliban fighters south of the Helmand provincial capital of Lashkar Gah, said Daud Ahmadi, the governor's spokesman.
Ahmadi says the authorities recovered a number of weapons, ammunition, motorbikes and other vehicles used by the Taliban. Two policemen were wounded.
Last week Taliban fighters launched several barrages of rocket and mortar fire into Lashkar Gah.
In Faryab province, militants killed five policemen, including a district police chief on Monday, said deputy governor Abdul Satar.
Associated Press reporter Noor Khan contributed to this report from Kandahar.
On the Net:
SERVE Afghanistan: http://www.serveafghanistan.org