An American who served as a monitor in the Darfur region of Sudan has returned to the United States to speak out about the atrocities he witnessed during his mission. He hopes his story will mobilize international efforts to stop what the United States has termed genocide.

Brian Steidle has been called an American witness to genocide.

The 28-year-old former Marine Captain returned to the United States last month after serving six months monitoring Darfur with an African Union mission. He shared his experiences with members of Congress and reporters.

Captain Steidle said his observations convince him that the Sudanese government, with support from Arab militias known as the Janjeweed, is clearly behind the atrocities he witnessed.

"This is not a tribal conflict between villages," he said. "This is not between one family and another family. This is a government-led military operation targeted at the African tribes in the Darfur region."

Captain Steidle offered evidence that the Sudanese government is using helicopter gunships to attack villages, something that Khartoum has denied. He showed a photograph depicting a helicopter gunship firing on a village, and held up ammunition that he says he found in a village that had been attacked by such aircraft.

"This is a flechette, which the helicopter gunships use on the villages. Every gunship carries four rocket pods, each rocket pod has about 20 rockets, and each rocket has about 500. These are only used to maim and kill people," Captain Steidle said.

Captain Steidle said his most frustrating moment was in December when the Sudanese government and the Janjaweed attacked the village of Labado, home to about 20,000 people. He said a Sudanese general refused to let him enter the village.

"Standing on the edge of the village with a Brigadier General in the Sudanese army standing next to me, watching this village burn to the ground," he said. "They had just attacked it. There was shooting in the village. It was being burnt to the ground in front of us. There was a solid stream of Arab militia, the Janjeweed, coming from one side with empty horses, camels, mules, and going back the other way fully loaded with stuff they had taken from the villagers."

Captain Steidle said the Sudanese general refused to stop the attack.

"We asked him what his mission was, he said it was to protect the civilians and to open the road to commercial traffic," he said. "I asked him why he did not stop what was going on. He said 'these are not my people. I do not have control over them.' He had 3,000 troops behind him."

His talk was illustrated with some particularly graphic photographs and slides. Several depicted skeletons, some with clothing still attached.

"This place here, there were bodies spread 20 meters that way, 30 meters that way," Captain Steidle said. "There was one area that was 50 meters by 50 meters, and you could not walk around without stepping on human bones. We have no idea how many people were there. They had been half buried like this. This individual here had been tied, pants pulled down around the knees presumably to be sexually assaulted."

Captain Steidle ended his talk on a hopeful note, saying what is happening in Darfur can be stopped with enough international support. He said what is needed is up to 50,000 troops on the ground, in every village and camp in Darfur.

Senator Sam Brownback, a Kansas Republican, who was among those who welcomed Captain Steidle, said U.N. Secretary-General Kofi Annan should be held accountable if the world body does not act soon on Darfur.

Mr. Brownback has introduced legislation calling for U.N. economic and diplomatic sanctions against those responsible for genocide in Sudan, including the Sudanese government.