United Nations Secretary-General Antonio Guterres has called for an immediate end to the violence, his spokesperson said on Twitter.
Guterres spoke with leaders of Sudan's army and paramilitary Rapid Support Forces, Egypt's president and the chair of the African Union Commission, the spokesperson said.Â
Rapid Support Forces claim to have seized the presidential palace, the army chief's residence and Khartoum international airport but the military said it was fighting back.
The RSF, which accused the army of attacking them first, also said they had taken over the airports in the northern city of Merowe and in El-Obeid in the west.
The situation on the ground was unclear. The army said it was fighting the RSF at sites the paramilitaries said they had taken. The army also said it had taken some RSF bases and denied that the RSF had taken Merowe airport.
The RSF says Egyptian troops who were in Sudan for joint exercises had handed themselves over to the group at Merowe, and it was coordinating with Egyptian authorities to guarantee their safety.
A major confrontation between the RSF and the army could plunge Sudan into widespread conflict as it struggles with economic breakdown and tribal violence, and could also derail efforts to move towards elections.
The clashes follow rising tensions between the army and the RSF over the integration of the RSF into the military and who should oversee the process. The disagreement has delayed the signing of an internationally-backed agreement with political parties on a transition to democracy.
On Saturday, the RSF accused the army of carrying out a plot by loyalists of former strongman President Omar Hassan al-Bashir - who was ousted in 2019 - and attempting a coup itself.
The RSF is headed by former militia leader General Mohamed Hamdan Dagalo, better known as Hemedti. He has been deputy leader of Sudan's ruling Sovereign Council headed by army General Abdel Fattah al-Burhan since 2019.
Gunfire could be heard in several parts of Khartoum and eyewitnesses reporting shooting in adjoining cities.
A Reuters journalist saw cannon and armoured vehicles deployed in the streets of the capital and heard heavy weapons fire near the headquarters of both the army and RSF.
Doctors groups say at least three civilians had been killed.
Clashes were also taking place at the headquarters of Sudan's state TV, said an anchor who appeared on screen.
The Sudanese armed forces spokesman told the Al Jazeera Mubasher television station that the army would respond to any "irresponsible" actions.
Brigadier-General Nabil Abdallah said there was a heavy presence of RSF troops at the TV headquarters in Khartoum.
Along with the UN, international powers - the US, Russia, Egypt, Saudi Arabia, and the European Union - all called for an end to the hostilities.
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