The United States is asking the African Union to investigate the shooting of a U.S. aid official Tuesday in Sudan's troubled Darfur region. The State Department did not assign blame for the incident, but said neither the Sudanese government nor Darfur rebels is doing enough to stop violence in the region.

The aid worker from the U.S. Agency for International Development (USAID) suffered a serious, though apparently not life-threatening injury, when struck in the face by a bullet as she rode in a convoy in western Darfur.

Secretary of State Condoleezza Rice said she was deeply saddened by the incident, and said the United States has asked the African Union, which has a 2,300 member observer force in the area, to launch an immediate investigation.

Ms. Rice said in a written statement the injured U.S. official was traveling in a clearly marked humanitarian vehicle on a road considered open by United Nations officials, and that the attack was "an apparent ambush" by unknown persons.

It was the first time in the history of the U.S. aid agency's Office of Foreign Disaster Assistance that one of its officials has been shot in the line of duty, though there have been numerous violent incidents involving relief workers in the region.

The Darfur conflict, pitting local rebels against Sudanese government troops and their Arab militia allies, the Janjaweed, erupted two years ago and has left tens of thousands of people dead and driven more than 1.5 million people from their homes.

In a Washington Post interview Tuesday, Sudanese First Vice President Ali Osman Taha said the Khartoum government had received an unfair share of the blame for the conflict, and that there must be more pressure on Darfur rebels to disarm before lasting peace can be achieved.

The remarks by Mr. Taha, who also rejected a U.S. assertion that killings in Darfur amounted to genocide, drew a sharp response from State Department Deputy Spokesman Adam Ereli.

He said the Sudanese government bears a large share of responsibility for what he termed the "horrific" conflict in Darfur and the "outrageous" atrocities committed there, which do constitute genocide.

Mr. Ereli said the rebels, too, have culpability and that neither side has been fully responsive to international calls for dialogue and a political settlement:

"Let's  be clear,” he said.  “It is incumbent on the government of Sudan, and the rebels, to take action to stop the violence. Neither one has done nearly enough. And for that reason the international community is engaged in Darfur, is engaged in trying to create the conditions so that the abused people of Darfur can return home, and leave camps, and be free from fear of the depredations of Janjaweed militia."

In the Washington Post interview, Vice President Taha called the Janjaweed "bandits" and said they were beyond the government's immediate control.

He said those who broke the law or committed atrocities would be punished, but that the Darfur rebels had to stop fighting and turn in their arms before the government could pursue the militiamen.

In his comments here, Spokesman Ereli said the United States is pressing to "beef up" the African Union force so that new atrocities do not occur, while working diplomatically for U.N. sanctions to keep pressure on both sides, and to assure accountability for crimes committed.