PUL-E-ALAM, Afghanistan—Taliban fighters with assault rifles shredded a U.S. aid group’s SUV with dozens of bullets Wednesday, killing three Western women and their Afghan driver amid an escalating militant onslaught against humanitarian workers in Afghanistan.

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 group whose workers were slain, the New York-based International Rescue Committee, announced it was suspending its Afghan humanitarian programs indefinitely.

The Taliban claimed responsibility for the killings, saying its fighters attacked two vehicles of “the foreign invader forces.”

“They were not working for the interests of Afghanistan and they belonged to those countries whose forces ... took Afghanistan’s freedom,” Taliban spokesman Zabiullah Mujahid told The Associated Press in a phone call from an undisclosed location.

Mujahid called the women spies, a frequent Taliban accusation against its targets.

The aid group identified the women killed in Logar province as Nicole Dial, 30, a dual Trinidadian-American citizen; Jacqueline (Jackie) Kirk, 40, of Outrement, Quebec; and Shirley Case, 30, of Williams Lake, British Columbia.

The 25-year-old driver, Mohammad Aimal, was from Kabul and had worked for the aid group since 2002.

“These extraordinary individuals were deeply committed to aiding the people of Afghanistan, especially the children who have seen so much strife,” said George Rupp, president of the International Rescue Committee.

Canadian Prime Minister Stephen Harper said the attack was a reminder of the Taliban’s brutality.

“This is obviously an outrage, a terribly brutal act, which I think should remind everybody of the brutality of the Taliban and the danger that everybody there faces,” Harper said.

The women were driving from the eastern province of Paktia to Kabul in a white SUV marked with IRC stickers, said Abdullah Khan, deputy counterterrorism director in Logar.

Five men armed with Kalashnikov assault rifles stepped out of a village area and fired at the two aid vehicles, Khan said, citing a report from an Afghan IRC employee wounded in the second vehicle. The women’s white SUV was hit by dozens of bullets, Khan said.

At the Pul-e-Alam hospital, IRC driver Abdurrahman Khan wept while helping load two of the victims’ bodies onto the back of a truck.

“They were here helping Afghan people,” he said. “They were not carrying weapons.”

All four victims suffered multiple bullet wounds, Dr. Mir Mabub Shah said as three female Afghan nurses shrouded the three dead women in white cloth before putting them in wooden coffins.