"We are ready for dialogue and cooperation," Abdalmahmoud Abdalhaleem, the  Sudanese ambassador, told Reuters news agency in an interview.

"We hope the US will reciprocate."

'Close adviser'

Gration, a decorated fighter pilot, was raised in Africa and is fluent in Swahili.

He is a close Obama adviser and often traveled with him during the presidential campaign last year.

They got to know each other when Obama visited Africa in 2006 while still a senator. During that trip they visited Darfur refugees in Chad, a neighbour of Sudan.

"I have worked closely and directly with General Gration for several years, and have traveled with him to refugee camps in Chad filled with those who were displaced by the genocide in Darfur," Obama said in the statement.

"He is a valued personal friend and I am pleased he has accepted this assignment."

Crisis time

The appointment comes at a time of deepening crisis in Sudan.

The country expelled 13 aid groups after the International Criminal Court charged President Omar Hassan al-Bashir with war crimes in Darfur, where 4.7 million people rely on foreign assistance for food, shelter and protection from fighting between rebels and government-backed forces.

"The government of Sudan's disastrous decision to expel humanitarian relief organizations leaves a void that will be filled by deprivation and despair and they will be held accountable for the lives lost," Obama said.

"I have made clear my intention to work with the international community to end the suffering," he added. "That means supporting the full, unobstructed deployment of the joint African Union/United Nations peacekeeping force and the negotiation of a political solution that will give the people of Darfur a meaningful voice."

Activists' satisfaction

Darfur activists welcomed the Gration announcement.

"He seems like a good choice. What is important is his experience, his gravitas and his close relationship with President Obama. I think all of those things will contribute greatly to his effectiveness," Jerry Fowler, president of the Save Darfur Coalition, told Reuters.

"We will be looking to see if he has the mandate and the authority to drive US policy on Sudan."