In dueling statements, the military and the R.S.F. blamed each other for the death of Mr. Abakar. General Hamdan even phoned the United Nations envoy to Sudan, Volker Perthes, to reiterate his denials, said a U.N. official who spoke on the condition of anonymity because they were not permitted to speak publicly. (That call in turn scotched speculation that General Hamdan, who has not been seen in public for many weeks, had been badly injured or killed.)

Humanitarian leaders have repeatedly warned that the crisis in Sudan could soon get much worse. The fighting has already displaced 2.2 million people and caused more than 950 deaths, according to the main Sudanese doctors’ union. Other estimates put the death count at more than 1,800.

The United Nations secretary general, António Guterres, said on Tuesday that he was “highly worried” by what he called “the increasing ethnic dimension of the violence” in the Darfur region, as well as by reports of widespread sexual violence.

David Miliband, head of the International Rescue Committee, an aid group, warned that Sudan was on course to become “the next Syria: the world’s largest humanitarian crisis both in terms of people in need and displacement to neighboring countries.”