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A summit of African leaders from war-torn Sudan's neighbours Thursday urged an end to the fighting, as UN experts reported a mass grave had been discovered in the country's Darfur region.

The International Criminal Court (ICC) meanwhile said it has opened a new probe into alleged war crimes in Sudan, adding that the escalating violence was a "matter of great concern"

While Cairo hosted the crisis meeting on the nearly three-month-old conflict, gun battles, explosions and the roar of fighter jets again shook the Sudanese capital Khartoum, residents told AFP.

At least 3,000 people have been killed and millions have fled their homes in the war between Sudan's rival generals, according to the Armed Conflict Location and Event Data Project.

Leaders of Egypt, Ethiopia, Eritrea, Chad, South Sudan, Central African Republic and Libya as well as of the African Union and Arab League met in Cairo to discuss the war and its regional impact.

The United Nations has warned that Sudan's conflict risks spiralling into "a full-scale civil war, potentially destabilising the entire region".

The ICC chief prosecutor Karim Khan said in a report to the UN Security Council Thursday that his office "can confirm that it has commenced investigations in relation to incidents occurring in the context of the present hostilities".

Khan said there had been a "wide range of communications" about alleged war crimes and crimes against humanity in Sudan since the fighting broke out in April.

In Cairo, Egyptian President Abdel Fattah al-Sisi hailed the "noble efforts" of Sudan's neighbours in "receiving hundreds of thousands of refugees with limited resources in an extremely difficult global economic situation".

He called on the international community "to honour the commitments" made last month when donors pledged $1.5 billion in aid -- less than half the estimated need for Sudan and its affected neighbours.

The summit called on both parties to secure corridors for urgently needed aid, even as Sudan's humanitarian crisis deepened.

Hundreds of people were seen queueing for drinking water in Wad Madani, 200 kilometres (130 miles) south of Khartoum, and a 24-hour electricity blackout hit most of the country.

'Denied decent burial'Ìý

Since the war erupted on April 15 between Sudan's army and paramilitary Rapid Support Forces, nearly 724,000 people have fled abroad, according to the International Organization for Migration.

The Chadian president, General Mahamat Idriss Deby Itno, said that within "just one week, Chad received more than 150,000 people, most of them women and children fleeing the violence".

The IOM says 240,000 people have escaped to Chad from Sudan's western region of Darfur, where entire towns have been ransacked.

The UN's human rights office OHCHR on Thursday reported new evidence of atrocities in the region.

It said the bodies of at least 87 people allegedly killed by the RSF and its allies between June 13-21 were buried in a mass grave in the West Darfur capital of El Geneina near the Chad border.

Some of the victims belonged to the non-Arab Massalit ethnic group, while seven women and seven children were among the dead, the office said, adding that the RSF were "denying those killed a decent burial".

'Humanitarian disaster'Ìý

Sudan's neighbours -- many already mired in economic and political crises -- have feared a widening regional spillover since the conflict began.

Sisi, a close ally of the Sudanese army, said more than 250,000 Sudanese had fled to Egypt, "joining around five million Sudanese citizens who have lived in Egypt for many years".

Human Rights Watch said Thursday that thousands more Sudanese are "stranded in dire humanitarian conditions" on the border, and urged Cairo to "rescind its entry visa rule" which was recently toughened.

Central African Republic's president, Faustin-Archange Touadera, warned of growing "small arms smuggling" across his country's "porous border" with Sudan.

"There are severe shortages in food and fuel," he said, warning of an impending "humanitarian disaster".

The Cairo summit follows multiple efforts to broker an end to the violence and repeated US and Saudi-brokered ceasefires that were all violated.

East African regional bloc IGAD held talks Monday in Addis Ababa calling on the warring parties to "sign an unconditional ceasefire".

The Sudanese army boycotted the gathering, after Khartoum's foreign ministry objected to Kenyan President William Ruto's leadership of the IGAD quartet because it accuses Nairobi of siding with the RSF.

Ethiopian Prime Minister Abiy Ahmed called Thursday for diplomatic efforts to "align with the IGAD-African Union process", while African Union Commission head Moussa Faki Mahamat urged a "political process without foreign interference".