The US government has said that Sudan's Transitional Military Council and the Rapid Support Forces should "desist from violence".
In a brief statement the US government also sent its condolences to the families of those who had died when the military crushed a pro-democracy demonstration.
A Sudanese official has denied that at least 100 people were killed by a paramilitary unit during protests, saying that the number was at most 46.
Doctors linked to the opposition on Wednesday said at least 100 people had been killed in the capital, Khartoum, amid pro-democracy protests.
They said 40 bodies were pulled from the River Nile in Khartoum on Tuesday.
Authorities had initially stayed quiet, but a health ministry official put the number at 46 early on Thursday.
Sudanese opposition activists have rejected an offer of talks from the country's military council, saying it cannot be trusted amid a violent crackdown on protesters.
Residents told the BBC they were living in fear in the capital.
The deputy head of the military council defended the violent suppression, claiming that the protesters had been infiltrated by rogue elements and drug dealers.
"We will not allow chaos and we will not go back on our convictions. There is no way back. We must impose the respect of the country by law," said Mohammed Hamadan - also known as Hemedti - on Wednesday.