Senate Majority Leader Bill Frist on Friday visited exhausted refugees who had fled into Chad from the Darfur region of western Sudan, where they have been under attack by an Arab militia. Frist called the crisis "one of the greatest humanitarian challenges of our time" and said the killing was "genocide."

The Tennessee Republican met with aid workers and toured the Touloum camp where 15,000 refugees live in dust-covered tents, fragile shelters vulnerable to rain and wind. The militia, known as the Janjaweed, has committed atrocities across Darfur that have displaced close to 1.5 million darker-skinned African villagers and left as many as 50,000 dead, according to human rights groups and aid workers.

Frist said that the violence was "specific to a group of people, with race playing a major role in intent. It is genocide." The House and Senate passed resolutions last month also declaring the situation a genocide and urged President Bush to seek a U.N. protection force.

One of the refugees in the camp, Amer Osman Adam Abdullah, said he spent 18 days walking from his village in Darfur to the border with Chad. He displayed frayed identification cards indicating that he had been a community leader. But now he helps organize the camp, a leader among the desperate. Using a bullhorn, he urged the refugees to clean up trash. In the pocket of his button-down shirt were two pens, which he said he used to record new arrivals at the camp.

Abdullah said no one would return to Darfur because of the lack of security there. But he thanked Frist and the United States for providing humanitarian aid and said, "If you go with us, we will go home," repeating the request for U.S. intervention voiced by other refugees.

Frist said a 30-day deadline set by the U.N. Security Council on July 30 gave the Sudanese government enough time to rein in the Janjaweed and end the crisis. Sudan has protested the U.S.-drafted resolution, which threatens the imposition of sanctions against the government, and instead has sought a 90-day deadline.

"The direct line between the government of Sudan and the Janjaweed and the raping, pillaging and murder is so direct that, with an order from the top, I am absolutely convinced it could stop within a week," Frist said. "If the president of Sudan says stop, he can stop it." He added that he would take his observations from his tour directly to Bush.

U.N. and U.S. officials say they have evidence that the Arab-led government in Khartoum has backed the Janjaweed militia and supplied it with weapons.

Frist said additional sanctions that would impose a travel ban on Janjaweed leaders and freeze their international bank accounts would have little practical effect and were largely symbolic. He said, however, that sending U.S. troops to the region was not yet an answer, either. British Prime Minister Tony Blair has said he has not ruled out a military response to the crisis if the attacks on civilians do not end.

A Sudanese army spokesman recently called the U.N. resolution "tantamount to a declaration of war," and on Wednesday 100,000 Sudanese participated in a government-backed rally in Khartoum, the capital, to protest any Western intervention.

Roger P. Winter, assistant administrator for the U.S. Agency for International Development, said it was important to keep international pressure on Sudan, a government that he said has a poor record of following up on its promises. "Unless we continue to pressure them, I don't think they will act for the well-being of the humanitarian situation there," Winter said. "This can no longer be considered the affairs of a sovereign state. The world has changed."

The Sudanese government has said that two African rebel groups, not the Janjaweed, started the war in Darfur. Officials assert that they are making good progress in disarming the Janjaweed, but aid workers say reports of fresh violence arrive every day.

Frist also met with Chad's president, Idriss Deby, at his peach-colored home in Biltine, about 500 miles northeast of the capital, N'Djamena. The two men discussed the plight of the 200,000 refugees from Darfur who have fled into Chad, putting pressure on an already desperately poor population. Deby said the situation would grow more dire if additional aid did not arrive.

"It will be a catastrophe. In our minds, we have what happened in Rwanda," Deby said, referring to the 1994 genocide in that country that left more than 500,000 people dead. "The international community has to act. Time is against us."

U.S. Senate Majority Leader Bill Frist listens to refugees at the Touloum camp in eastern Chad. About 200,000 people have fled violence in western Sudan.