Nobel Peace Prize-winning relief agency Medecins Sans Frontieres (search) suspended operations in Afghanistan on Thursday, a day after five of its aid workers were killed in an ambush claimed by the former Taliban regime.

The assault on the Medecins Sans Frontieres operation in northern Afghanistan's Badghis province was the deadliest since the radical Islamic militia was ousted in late 2001 and cast new doubt on the country's readiness for national elections slated for September.

The foreign aid workers — a Norwegian doctor, a Dutch logistician and the Belgian project coordinator — and their Afghan driver and translator were killed when attackers on a motorcycle shredded a four-wheel-drive Toyota painted with the organization's red logo with assault rifles and grenades.

"For the time being, our activities will be suspended nationwide," MSF spokeswoman Vicky Hawkins told a news conference. "In the coming weeks we will analyze this event in-depth, but for the moment our priority is to take care of those most affected by this tragedy."

MSF employs 80 expatriates and 1,400 local people in twelve Afghan provinces, and the suspension reflected an immediate increase in fear that an insurgency that has already severely limited operations by relief agencies in the south and east of the country could be spreading.

The organization was pulling all of its foreign workers back to Kabul, leaving local staff in place to perform only life-saving activities, said Bas Tielens, a spokesman in Amsterdam.

Tielens said other projects were suspended until at least Monday. He wouldn't say if the group might pull out of some areas altogether.

"We're going into the process of dealing with the shock and re-evaluating," he said.

A pullback could have grave implications. MSF is one of the most professional non-governmental relief agencies and often sets the trend for others. It has been in Afghanistan since 1979, where it provides basic health care and support to hospitals as well as programs for tuberculosis and mental illness. The organization, also known as Doctors Without Borders, won the Nobel Peace Prize in 1999.

Danish aid group DACAAR, the only other major international aid group working in Badghis, said it had ordered staff digging wells and helping farmers in the region to stay off the roads until it can re-assess security.

"We'd adjusted ourselves to a particular level of criminal risk" said Gorm Pedersen, head of the group's operations here. "That we could handle, but now it may be becoming political. It's most worrying."

The United Nations halted voter registration in Badghis, but said its other operations would continue for now.

President Hamid Karzai (search) expressed sorrow about the attack before leaving Thursday for the United States. But despite daily assaults against soldiers and election workers, Karzai claimed poverty worried ordinary Afghans more.

"I think we are quite all right with security," Karzai said. "We have incidents, sure, we must reduce them. But this is not an alarming thing."

But the United Nations said in a statement it was "deeply shocked and outraged" and that the attack confirmed that security had "evolved negatively" in recent months. It called for more foreign troops to help Afghans provide security.

The slain foreigners were identified as Egil Tynaes, a 63-year-old doctor from Norway, logistician Willem Kwint, a 40-year-old Dutchman, and project coordinator Helene de Beir, 30, from Belgium.

Police investigating the incident said a farmer saw gunmen ambush the group Wednesday afternoon in a desert area near Khair Khana, a village 550 kilometers (340 miles) west of Kabul.

"Two men on a motorbike stopped the car and opened fire with Kalashnikovs," Badghis police chief Amir Shah Naibzada said. "He gave a detailed description of the attackers."

In Kabul, MSF officials choked back emotion as they told reporters how colleagues found the vehicle after the victims missed a radio check. Bullets had shattered the windows and grenade shrapnel was lodged in one side. The attackers disconnected the radio, but stole nothing.

The group said it had experienced no problems or received any threats in Badghis, an area considered among the safest for aid workers.

Mullah Abdul Hakim Latifi, a purported spokesman for the Taliban, called The Associated Press on Wednesday and said the militia staged the attack. He threatened more attacks and claimed that "international aid workers were working for the policy of America."

Taliban rebels and their Al Qaeda allies have killed at least 33 aid workers since March last year, most of them Afghans.

Last November, gunmen killed Bettina Goislard, a 29-year old worker for the United Nations High Commission for Refugees, in the eastern city of Ghazni, the first foreign U.N. staff member slain since the fall of the Taliban.

Goislard's killing prompted a pullback of international staff and the suspension of refugee assistance across the south and east which only resumed in March.