Clooney, 45, and Nobel Peace Prize recipient Elie Wiesel spoke with U.N. officials Thursday in New York asking them to stop the killing in the Sudanese region, People magazine reported.
"The United States has called it genocide," the former "ER" star told the Security Council. "For you it's called ethnic cleansing. But make no mistake, it is the first genocide of the 21st century. And if it continues unchecked, it will not be the last."
Clooney recently spent five days in the region with his father, retired newsman Nick Clooney, and has since been urging Washington officials to become more involved in the situation.
People said Wiesel echoed Clooney's plea to the U.N., comparing the violence in Darfur with the genocide that occurred in nearby Rwanda.
"Nobody did anything," said Wiesel. "It remains a mark of shame for the (United Nations) and the whole world."