Sudan's main paramilitary group said it had seized the presidential palace, the army chief's residence and Khartoum international airport on Saturday in an apparent coup attempt but the military said it was fighting back, Reuters reports.
The Rapid Support Forces (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.
A major confrontation between the RSF and the army could plunge Sudan into widespread civil conflict as it struggles with economic breakdown and tribal violence.
The RSF accused the army of carrying out a plot by loyalists of ousted President Omar Hassan al-Bashir and attempting a coup itself.
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 said clashes had occurred in residential neighbourhoods and at least three civilians had been killed.
Clashes were also taking place at the headquarters of Sudan's state TV, an anchor who appeared on screen briefly said.
There was a heavy exchange of gunfire in Merowe, eyewitnesses told Reuters.
