The United Nations Friday called for an investigation into a recent attack on aid workers delivering assistance in the Western Upper Nile area of southern Sudan.

The spokesman for the United Nations Humanitarian Coordinator for Sudan says unidentified militiamen used machine guns, mortars and rocket-propelled grenades to fire on eight aid workers during a relief operation in the Western Upper Nile town of Nimnim.

The aid workers were not injured. They were bringing supplies to more than 13,000 people in Nimnim when they were attacked Friday, February 20. The spokesman, Ben Parker, says relief operations have been suspended in the area, affecting about 30,000 people.

Mr. Parker says the United Nations is calling on the Sudanese government and the Sudan People's Liberation Army, which controls Nimnim and the surrounding area, to investigate the attack. He called the incident a war crime, according to a recently-passed Security Council resolution. "There was no way in which this attack was crossfire, or an accident, or part of a more general destabilization," he said. "This was a very targeted military attack from the ground, and in that sense, it's something we've very, very rarely seen before."

Meanwhile, in the troubled western Sudan region of Darfur, aid workers are still struggling to reach vulnerable civilians caught in fighting between government forces and other rebel groups.

"In Darfur, we have access to about 25 or 30 percent of the displaced people in that war-affected region," said Mr. Parker. "There are still challenges with regard to bringing in new supplies. Some of the major arteries into the region are still not 100 percent secure, and there are still access and security problems within the region."

The medical aid agency, Doctors Without Borders, released a statement Thursday, saying displaced people in war-torn Darfur are desperately short of food and water.

In the aid agency's words, without urgent medical, food and sanitation assistance, these people's lives are threatened.

Up to one million people have been affected by the Darfur war, which has been raging for a year among rebels, government troops and Arab militias, many say are aligned with the government.

It is only recently that the Sudanese government has opened up the area to aid workers, but the lack of security still makes many places inaccessible.