At first light on Sunday, three young women walked into a scrubby field just outside their refugee camp in West Darfur. They had gone out to collect straw for their family's donkeys. They recalled thinking that the Arab militiamen who were attacking African tribes at night would still be asleep. But six men grabbed them, yelling Arabic slurs such as "zurga" and "abid," meaning "black" and "slave." Then the men raped them, beat them and left them on the ground, they said.

"They grabbed my donkey and my straw and said, 'Black girl, you are too dark. You are like a dog. We want to make a light baby,' " said Sawela Suliman, 22, showing slashes from where a whip had struck her thighs as her father held up a police and health report with details of the attack. "They said, 'You get out of this area and leave the child when it's made.' "

Suliman's father, a tall, proud man dressed in a flowing white robe, cried as she described the rape. It was not an isolated incident, according to human rights officials and aid workers in this region of western Sudan, where 1.2 million Africans have been driven from their lands by government-backed Arab militias, tribal fighters known as Janjaweed.

Interviews with two dozen women at camps, schools and health centers in two provincial capitals in Darfur yielded consistent reports that the Janjaweed were carrying out waves of attacks targeting African women. The victims and others said the rapes seemed to be a systematic campaign to humiliate the women, their husbands and fathers, and to weaken tribal ethnic lines. In Sudan, as in many Arab cultures, a child's ethnicity is attached to the ethnicity of the father.

"The pattern is so clear because they are doing it in such a massive way and always saying the same thing," said an international aid worker who is involved in health care. She and other international aid officials spoke on condition of anonymity, saying they feared reprisals or delays of permits that might hamper their operations.

She showed a list of victims from Rokero, a town outside of Jebel Marra in central Darfur where 400 women said they were raped by the Janjaweed. "It's systematic," the aid worker said. "Everyone knows how the father carries the lineage in the culture. They want more Arab babies to take the land. The scary thing is that I don't think we realize the extent of how widespread this is yet."

Another international aid worker, a high-ranking official, said: "These rapes are built on tribal tensions and orchestrated to create a dynamic where the African tribal groups are destroyed. It's hard to believe that they tell them they want to make Arab babies, but it's true. It's systematic, and these cases are what made me believe that it is part of ethnic cleansing and that they are doing it in a massive way."

Secretary of State Colin L. Powell flew to the capital, Khartoum, on Tuesday to pressure the government to take steps to ease the humanitarian crisis in Darfur. U.S. officials said Powell may threaten to seek action by the United Nations if the Sudanese government blocks aid and continues supporting the Janjaweed. U.N. Secretary General Kofi Annan is due to arrive on Khartoum this week.

The crisis in Darfur is a result of long-simmering ethnic tensions between nomadic cattle and camel herders, who view themselves as Arabs, and the more sedentary farmers, who see their ancestry as African. In February 2003, activists from three of Darfur's African tribes started a rebellion against the government, which is dominated by an Arab elite.

Riding on horseback and camel, the Janjaweed, many of them teenagers or young adults, burned villages, stole and destroyed grain supplies and animals and raped women, according to refugees and U.N. and human rights investigators. The government used helicopter gunships and aging Russian planes to bomb the area, the U.N. and human rights representatives said. The U.S. government has said it is investigating the killings of an estimated 30,000 people in Darfur and the displacement of the more than 1 million people from their tribal lands to determine whether the violence should be classified as genocide.

The New York-based organization Human Rights Watch said in a June 22 report that it investigated "the use of rape by both Janjaweed and Sudanese soldiers against women from the three African ethnic groups targeted in the 'ethnic cleansing' campaign in Darfur." It added, "The rapes are often accompanied by dehumanizing epithets, stressing the ethnic nature of the joint government-Janjaweed campaign. The rapists use the terms 'slaves' and 'black slaves' to refer to the women, who are mostly from the Fur, Masalit and Zaghawa ethnic groups."

Despite a stigma among tribal groups in Sudan against talking about rape, Darfur elders have been allowing and even encouraging their daughters to speak out because of the frequency of the attacks. The women consented to be named in this article.

In El Fasher, the capital of North Darfur, about 200 miles east of Geneina, Aisha Arzak Mohammad Adam, 22, described a rape by militiamen. "They said, 'Dog, you have sex with me,' " she said. Adam, who was receiving medical treatment at the Abu Shouk camp, said through a female interpreter that she was raped 10 days ago and has been suffering from stomach cramps and bleeding. "They said, 'The government gave me permission to rape you. This is not your land anymore, abid, go.' "

Nearby, Ramadan Adam Ali, 18, a frail woman, was being examined at the health clinic. She was pregnant from a rape she said took place four months ago. She is a member of the Fur tribe and has African features.

"The man said, 'Give me your money, slave,' " she said, starting to cry. "Then I must tell you very frankly, he raped me. He had a gun to my head. He called me dirty abid. He said I was very ugly because my skin is so dark. What will I do now?"

In Tawilah, a village southeast of El Fasher, women and children are living in a musty school building. They said it was too dangerous to leave and plant food.

Fatima Aisha Mohammad, once a schoolteacher, stood in a dank classroom describing what happened to her three weeks ago, when she left the school to collect firewood.

"Very frankly, they selected us ladies and had what they wanted with us, like you would a wife," said Mohammad, 46, who has five children. "I am humiliated. Always they said, 'You are nothing. You are abid. You are too black.' It was disgusting."

During a recent visit, government minders warned people at the school to stop talking about the rapes or face beatings or death. Minders also were seen handing out bribes to keep women from speaking to foreign visitors. But those at the school spoke anyway. A group of people handed a journalist two letters in Arabic that listed 40 names of rape victims, and wanted the list to be sent to Sen. Sam Brownback of Kansas and Rep. Frank R. Wolf of Virginia, Republicans who were touring the region and pressing the government to disarm the Janjaweed.

"I was sad. I am now very angry. Now they are trying to silence us. And they can't," Mohammad said. "What will people think of all of us out here? That we did this to ourselves? People will know the truth about what is happening in Darfur."

Later that day in Tawilah's town center, Kalutum Kharm, a midwife, gathered a crowd under a tree to talk about the rapes. Everyone was concerned about the children who would be born as a result.

"What will happen? We don't know how to deal with this," Kharm lamented. "We are Muslims. Islam says to love children no matter what. The real problem is we need security. We don't trust the government. We need this raping to stop."

Aid workers and refugees in Geneina said that despite an announcement last week by Sudan's president, Lt. Gen. Omar Hassan Bashir, that the Janjaweed would be disarmed, security had not improved. Janjaweed dressed in military uniforms and clutching satellite phones roamed the markets and the fields, guns slung over their shoulders. Last week, the Janjaweed staged a jailbreak and freed 13 people, aid workers said. They also killed a watermelon salesman and his brother because they did not like their prices, family members of the men said.

A government official, speaking with a reporter, described the rapes as an inevitable part of war and dismissed accusations by human rights organizations that the attacks were ethnically based.

In Geneina, two women told their stories while sitting in front of their makeshift straw shelter. One of the women, a thin 19-year-old with dead eyes, moved forward.

"I am feeling so shy but I wanted to tell you, I was raped too that day," whispered Aisha Adam, the tears rushing out of her eyes as she covered her face with her head scarf. "They left me without my clothing by the dry riverbed. I had to walk back naked. They said, 'You slave. This is not your area. I will make an Arab baby who can have this land.' I am hurting now so much, because no one will marry me if they find out."

Sitting on mats outside the shelter, Sawela Suliman's father talked with village elders about what to do if his daughter became pregnant.

"If the color is like the mother, fine," he said as a crowd gathered to listen. "If it is like the father, then we will have problems. People will think the child is an Arab."

Then his daughter looked up.

"I will love the child," she said, as other women in the crowd agreed. "But I will always hate the father."

Then the rains came. They pounded onto the family's frail shelter, turning their roof into a soggy and dripping clump of straw. Suliman started to shiver as the weather shifted from steaming hot to a breezy rain. She will no longer leave the area of her hut to collect straw. She will stay here, hiding as if in prison, she said, and praying that she is not pregnant.

In Sudan's Darfur region, a woman who was raped by Janjaweed militiamen shows her scars. Aid workers say such rapes are systematic. A mother in a village in Darfur warns her daughter not to leave the schoolhouse where other women are hiding from the militiamen.Hawa Hussein, 35, left, and Hawa Juma, 25, of Kas, Sudan, were raped by militiamen. Hussein is pregnant as a result.