KHARTOUM, Sudan -- Nine Sudanese relief workers for an American aid agency were killed during clashes between warring tribes in western Sudan, an agency official said Sunday.

Andrew Natsios, head of the United States Agency for International Development, told reporters at the American Embassy in Khartoum that the nine workers were killed a week ago while transferring aid to displaced Sudanese in camps in the country's western Darfur region.

It was unclear who killed the workers.

Natsios urged the Sudanese government to guarantee the safety of USAID workers and volunteers traveling to camps in Darfur, some 640 miles west of Khartoum. He said the recent fighting in Darfur had created more than 600,000 refugees.

"We are concerned about the situation there," Natsios said.

Earlier, Sualif el-Deen Salih, the head of Sudan's governmental Humanitarian Aid Agency, which supervises all nongovernment and international organizations, acknowledged for the first time the seriousness of the situation in Darfur.

Recent tribal clashes there killed more than 100 people and scattered thousands of people from 15 villages into the wilderness fearing for their lives, press reports in Khartoum said last week.

Darfur, on the border with Chad, is home to some 80 tribes and ethnic groups divided between nomads of Arab origin and farmers of African origin.

Nearly a fifth of Sudan's 30 million people live in the region, one of the country's least developed, where cycles of drought and desert creep have shrunk its vast grazing areas and spurred friction among nomads and farmers.

The situation worsened earlier this year when a Darfur group demanding self-determination for the region attacked Sudanese government troops. Last month, the government and the Darfur Liberation Army agreed to a 45-day cease-fire.