UN Secretary General Antonio Guterres said Sudan is on the brink of a "full-scale civil war" that could destabilise the entire region, in a statement released after an air strike on a residential area killed about two dozen civilians.
The UN chief "remains deeply concerned that the ongoing war between the armed forces has pushed Sudan to the brink of a full-scale civil war, potentially destabilising the entire region", his spokesman Farhan Haq said.
Mr Guterres also condemned the air strike in Khartoum's sister city Omdurman on Saturday.
The Sudanese Ministry of Health reported "22 dead and a large number of wounded among the civilians" from the assault in the district of Dar Al Salam.
The attack was one of the deadliest in nearly three months of fighting around the capital and elsewhere in Sudan between the army and the paramilitary Rapid Support Forces (RSF). Last month, an air strike killed at least 17 people including five children in Khartoum.
The RSF blamed the military for Saturday's air strike as well as other attacks on residential areas in Omdurman, where residents say fighting has raged between the warring factions. The military has reportedly attempted to cut off a crucial supply line for the paramilitary force there.
A video posted by the health ministry on Facebook showed apparently lifeless bodies after the air strike, including several women. The narrator says residents "counted 22 dead".
The RSF blamed the military for the attack in Omdurman. The military denied the accusation, saying in a statement on Sunday that its air force did not carry out any airstrikes in the city that day.
Two Omdurman residents said it was difficult to determine which side was responsible for the attack. They told Associated Press that military aircraft have repeatedly targeted RSF troops in the area and the paramilitary force has used drones and anti-aircraft weapons against the army.
At the time of the attack early on Saturday, the military was targeting the RSF, which took people’s houses as shields, and the RSF fired anti-aircraft rounds at the attacking warplanes, said Abdel-Rahman, one of the residents who asked that only his first name be used over concern for his safety.
“The area is like a hell ... fighting around the clock and people are not able to leave,” he said.
Sudan, in north-east Africa, borders several impoverished countries that have a history of unrest.
About 3,000 people have been killed in the conflict, survivors have reported a wave of sexual violence and witnesses have spoken of ethnically targeted killings. There has been widespread looting and the UN warned of possible crimes against humanity in the Darfur region.
Nearly three million people have been displaced, including almost 700,000 who have fled to neighbouring countries, the International Organisation for Migration estimates.
"There is an utter disregard for humanitarian and human rights law that is dangerous and disturbing," Mr Haq said.
The UN and African blocs have warned of an "ethnic dimension" to the conflict in the western region of Darfur, where the US, Norway and Britain have blamed the RSF and allied militia for most of the widespread violations.
Mr Haq expressed support for efforts by the African Union and the East African bloc IGAD to help bring Sudan's crisis to an end.
On Monday leaders of Ethiopia, Kenya, Somalia and South Sudan — IGAD members handling the Sudan file — are to meet in Addis Ababa.
Sudan's army chief Abdel Fattah Al Burhan and RSF commander Gen Mohammed Dagalo have been invited but neither has confirmed they will attend.
Numerous ceasefires in the war have been announced and ignored.
The fighting broke out 18 months after the two generals led a military coup in October 2021 that toppled a Western-backed civilian transitional government. The conflict dashed Sudanese hopes of a peaceful transition to democracy after a popular uprising forced the military removal of longtime autocrat Omar Al Bashir in April 2019.
Agencies contributed to this report.