Amnesty International and Human Rights Watch during a weeklong mission to South Kordofan found at least 26 civilians were killed during Sudanese airstrikes in the area. Sudan blames South Sudanese rebels for much of the violence amid accusations charging Khartoum with an ethnic-cleansing campaign in the southern state.
Victoria Nuland, a spokeswoman for the U.S. State Department, called on both sides to let humanitarian groups gain access to the region. She also reminded Sudan's President Omar al-Bashir of his commitment to a cease-fire in the region.
"The United States is deeply concerned about reports of continued Sudanese air force bombings of civilian areas in South Kordofan, despite President Bashir's announcement of a unilateral two-week cease-fire last Tuesday," she told reporters during her regular news briefing.
The United Nations said it had evidence to suggest much of the violence is ethnically motivated. Khartoum counters that many of the allegations are based on hearsay.
A spokesman for the Sudanese military said the situation in South Kordofan is stable and the claims made by the two human rights organizations are false, the state-run Sudan News Agency reports.