Washington is "very concerned" about allegations of ethnic killings in South Kordofan state in South Sudan, a government spokesman said. The Satellite Sentinel Project, founded by human rights activist John Prendergast and Hollywood star George Clooney, said it had visual evidence that it claimed showed mass graves in South Kordofan. The United Nations welcomed South Sudan as the newest member Thursday. Its entrance followed its inaugural Independence Day July 9, which came as a result of a peace deal reached in 2005 that ended Sudan's civil war. Despite independence and a series of peace initiatives, the international community has expressed concern over violence along the border between Sudan and South Sudan. "This evidence demonstrates the urgent need for a full-scale international investigation into the violence in South Kordofan, and underlines the imperative to protect civilian populations from their own government in Khartoum," Prendergast said in a statement. Susan Rice, Prendergrast's former boss and U.S. envoy to the United Nations, said during U.N. Security Council deliberations on Sudanese issues that the situation there was fragile. Mark Toner, a spokesman for the U.S. State Department, said Washington couldn't officially confirm the mass grave allegations but stressed it was "extremely important" to get access to the region. "I would just say that we remain very concerned about allegations of targeted and ethnic-based killings and other human rights abuses in Southern Kordofan, and we call on all parties there to take immediate steps to end the violence and protect innocent civilians," he told reporters.