Khartoum had already announced on Tuesday that it was freezing relations with IGAD for inviting paramilitary chief Mohamed Hamdan Daglo — at war with army chief Abdul Fattah al Burhan for nine months — to a summit in Uganda that discussed the Sudan conflict.
In a statement, Sudan's foreign ministry said Burhan sent a letter on Saturday to President Ismail Omar Guelleho of Djibouti, current chair of the bloc, "to inform him of the Sudanese government decision suspending its membership in this organization."
Sudan "is facing one of the fastest unfolding crisis globally," the United Nations humanitarian agency, OCHA, says, with more than 7.4 million people displaced and more than half the population needing humanitarian assistance.
The Armed Conflict Location & Event Data Project, an analyst group, puts the death toll at more than 13,000.
At its summit on Thursday, IGAD reiterated its call for "an immediate and unconditional cease-fire" in the "unjust war affecting the people" of Sudan.
The bloc also expressed "continued readiness to offer its good offices to facilitate an all-inclusive peace process," and again called for a face-to-face meeting between the two sides.
A final communique from the extraordinary assembly gave the generals two weeks to meet.
Sudan's foreign ministry said it was this item, placed on the agenda without Sudan's approval, which led it to the further step of suspending its membership.
The ministry also accused the summit communique of violating Sudan's sovereignty and "wounding the sentiments of victims of rebel militia atrocities," a reference to Daglo's Rapid Support Forces, RSF.
Both sides have been accused of war crimes, including the indiscriminate shelling of residential areas, torture and arbitrary detention of civilians.
The RSF has also been accused of ethnically motivated mass killings, rampant looting and rapes.
IGAD, in parallel with the United States and Saudi Arabia, has repeatedly attempted to mediate between the two warring generals, but to no avail.
Daglo toured several African capitals from late December in his first foreign trip since the start of the war last April. It is part of a strategy analysts say is a bid for international legitimacy and is likely linked to the United Arab Emirates.
In Addis Ababa, Daglo signed a declaration with Sudan's former civilian Prime Minister Abdalla Hamdok, who also attended the IGAD summit.
The RSF appears to have been gaining new ground in recent months, with little resistance from the army.
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