Aug. 14 (Bloomberg) -- The United Nations condemned the ``brutal murder'' of four international aid workers in Afghanistan yesterday and said relief efforts in the country are becoming harder because of such attacks.

``Nothing can justify such an attack on people whose lives are devoted to helping the people of Afghanistan,'' John Holmes, the UN's humanitarian affairs chief, said in a statement after four members of the International Rescue Committee were killed by gunmen in the country's northeast.

It was the deadliest single attack against the aid community in Afghanistan this year, taking the total number of workers killed in 2008 to 19, Holmes said. The IRC suspended operations in Afghanistan indefinitely, joining other charitable groups cutting services in war-torn countries.

Three female IRC workers, a British-Canadian, a Canadian and a Trinidadian-American, and their Afghan driver, were traveling from an office in Gardez when they were attacked in the province of Logar, about an hour south of Kabul, said Michael Kocher, vice president of international programs for the New York-based organization.

``They were murdered in broad daylight,'' Kocher said in a telephone interview from New York.

`Hail of Bullets'

The IRC workers were unarmed and traveling in clearly marked vehicles when they were ambushed by gunmen who ``sprang out'' and unleashed a ``hail of bullets,'' Kocher said. Another Afghan driver was seriously wounded. The names of the victims weren't released.

An anti-government group called a radio station in Kabul several hours after the attack to claim responsibility, Kocher said. The IRC has 11 international members and 530 local residents working in Afghanistan.

``I condemn this cowardly attack in the strongest possible terms and urge the authorities to leave no stone unturned in the search for the perpetrators,'' said Kai Eide, the UN's top envoy to Afghanistan, in a statement.

The ambush comes as the Afghan government is facing a revitalized insurgency by supporters of the Taliban regime that was ousted by U.S.-led forces in late 2001.

Nan Dale, the U.S. executive director for Action Against Hunger/Action Contre la Faim, which had two workers kidnapped in Afghanistan last month, said humanitarian workers are being singled out. The group stopped operations in Sri Lanka, where 17 workers were killed in 2006, Dale said last month.

Doctors Without Borders withdrew its 97 international staff members from Somalia after three of its workers were killed by a roadside bomb in January. Eight members of other groups were kidnapped this month.

Killings, kidnappings and violence against aid workers around the world have more than doubled in the past five years, according to a draft of a report to be published by London-based Overseas Development Institute in the next three months. The average annual number of incidents rose to 76 between 2003 and 2007, compared with 35 in the previous five-year period.

``Humanitarian workers should be protected by the principles of neutrality and impartiality and must not be targeted, as they appear to be at present in Afghanistan,'' said Holmes.

To contact the reporters on this story: Chris Dolmetsch in New York at cdolmetsch@bloomberg.net; Ed Johnson in Sydney at ejohnson28@bloomberg.net.

Last Updated: August 13, 2008 20:05 EDT