A Wednesday attack that killed three Western aid workers in Afghanistan raises concerns that the Taliban is attempting to force the expulsion of all foreign humanitarian workers from the troubled country.

"This was the worst attack in many years and is a major escalation of hostilities," says Sayed Rahim Satar, vice chairman of the Afghan NGO Coordinating Bureau.

The assault signals a shift in the Taliban's strategy toward a policy of direct confrontation with nongovernmental organizations (NGOs), says Waliullah Rahmani of the Kabul Center for Strategic Studies.

"This appears to be the beginning of a new approach," he says, "to surround Kabul and eliminate any foreign or government presence in the area."

Gunmen ambushed a vehicle carrying British-Canadian Jacqueline Kirk, Canadian Shirley Case, Trinidadian-American Nicole Dial, and Afghan employees of the International Rescue Committee (IRC), a New York-based NGO, in Logar, a province adjacent to Kabul.

The aid workers, who had been helping to establish schools and train teachers in the region, were traveling back from outer provinces to Kabul when the incident occurred.

The Taliban claimed that the women were spies. Spokesman Zabiullah Mujahid told the Associated Press that they "were not working for the interests of Afghanistan."

In response to the killings, the IRC suspended its operations after more than 20 years of work in the country.

Now, aid organizations are warning that if aid workers go, badly needed social services will dry up and cripple the Afghan government, strengthening the Taliban's position.

19 aid workers killed this year

The assault comes during a year of rising violence in general and against aid groups in particular.

Three members of the US-led coalition were killed Thursday by an explosion targeting international troops, said the US-led coalition, which did not immediately release any details.

Nineteen aid workers have been killed this year, a figure that already exceeds the number of aid workers killed in 2007, according to the Agency Coordinating Body for Afghan Relief (ACBAR), an umbrella group of various Afghan NGOs.

ACBAR reports that NGOs have been attacked 84 times this year and 21 times in July alone.

In April, the Afghan NGO Safety Office reported that attacks against aid organizations jumped nearly fivefold compared with 2007.

Insurgents regularly intimidate NGO employees and obstruct their work, accusing foreigners of being spies and aiding the central government. Humanitarian work in some regions of the country is extremely dangerous, and many agencies have curtailed their activities in these areas.

However, until this year, there were relatively few high-profile attacks against NGOs.

In 2003, Taliban fighters ambushed a convoy of the International Committee of the Red Cross and murdered one foreign aid worker in the southern province of Uruzgan.

The incident sparked the evacuation of many NGOs from the restive southern provinces to Kabul.

Last year, the Taliban abducted nearly 20 South Korean missionaries and murdered two of them before releasing the rest for a large ransom.

In January, insurgents kidnapped an American NGO worker, who is officially listed as missing although authorities fear that she is dead. But despite such incidents, insurgents have largely avoided large-scale attacks against the foreign aid workers.

Taliban strategy: encircle Kabul

This spring, the Taliban announced that encircling Kabul would be its main strategic objective in 2008. The number of insurgent-initiated attacks in the provinces bordering Kabul jumped by 50 percent this year, according to data from the Vigilant Strategic Services of Afghanistan, a security analyst firm.

The Taliban, a ragtag force that often employs crude methods and weaponry, is usually no match for Coalition forces.

In recent months, militants switched to soft targets and masterminded a series of high-profile assaults, starting with a raid on a luxury hotel in Kabul in January.

"The Taliban know that they can gain a lot from high-profile attacks against the aid organizations and other noncombatants," Mr. Rahmani says. "A simple ambush like this gets the whole world talking, more than any drawn out military battle with Coalition forces" could.

NGOs' key role

Afghanistan's weak central government and lack of infrastructure means that NGOs are one of the main providers of social services such healthcare, education, and development.

By striking at aid organizations, insurgents hope to exploit the government's inability to stand on its own.

"This is very dangerous for Afghanistan," says Rahmani. "Aid agencies have an important role in strengthening the Afghan state and improving its public image.

But without such agencies, the government becomes very weak, unable to provide basic services for its people – a situation that the Taliban will exploit readily."

As instability and violence spread north from Kandahar and Helmand to the areas around Kabul, aid agencies may be pushed farther north.

With the central government's presence already minimal in some of the districts just south of Kabul – a recent US intelligence estimate declared that the national government controls only about 30 percent of the country – the Taliban will be left to fill the vacuum.

For many in the aid community, Wednesday's attacks came as a painful reminder that security is and will be for some time the primary concern of humanitarian agencies.

"I think things are going to only get worse, especially with the [2009 Afghan presidential] elections coming up," says an employee at a prominent international NGO who spoke on condition of anonymity.

"In order for us to operate, the local communities must guarantee our safety," says the employee. "If not, aid work becomes very difficult."