The humanitarian crisis in Darfur has been obscured as the Sudanese government and its allies focus on protecting the country's president from an international arrest warrant for war crimes, a Sudanese opposition leader complained to a visiting U.S. envoy on Friday. President Barack Obama's special envoy to Sudan arrived in Khartoum on Thursday, on his first visit that will take him also to the country's western Darfur region and southern Sudan. J. Scott Gration said he is in Sudan "to look, listen and learn." He is expected to meet officials and representatives from different parts of the country. The visit follows Sudanese President Omar al-Bashir's decision to expel 13 foreign aid groups in retaliation for the arrest warrant against him from the International Criminal Court. It's unclear if Gration will have a meeting with al-Bashir.