Victoria Nuland, a spokeswoman for the State Department, said there were credible reports that as many as 70 civilians were killed in Darfur in fighting between Sudanese and rebel forces last month.
"We are also appalled by the recent deadly attack on a patrol of the United Nations – African Union hybrid operation in Darfur," she said.
Four Nigerian peacekeepers were killed and eight other UNAMID soldiers were injured in an ambush in Darfur this week.
Nuland said Washington condemns attacks on peacekeepers in the "strongest possible terms," noting the Sudanese government had an obligation to investigate the attacks and bring the perpetrators to justice.
She said the attacks on UNAMID and a deteriorating security situation there suggested Khartoum wasn't interested in upholding its pledges to implement peace agreements related to Darfur.
A mid-September advisory from the State Department described the threat level in Darfur as "critical."