Sudan's main paramilitary group and the regular armed forces exchanged gunfire in Khartoum and elsewhere in the country on Saturday in an apparent struggle for control.
The army rejected assertions by the Rapid Support Forces (RSF) that they had seized the presidential palace, the army chief's residence and airports in Khartoum and the northern city of Merowe.
Army chief General Abdel Fattah Al-Burhan told Al Jazeera TV: "We think if they are wise they will turn back their troops that came into Khartoum. But if it continues we will have to deploy troops into Khartoum from other areas."
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The RSF leader, General Mohamed Hamdan Dagalo, better known as Hemedti, called Burhan a "criminal" and a "liar".
"We know where you are hiding and we will get to you and hand you over to justice, or you die just like any other dog," he said in an interview with the station.
With conflicting versions of events given by the two sides, the situation on the ground was unclear.
The RSF said the army had attacked it first, while the army said it was fighting the RSF at sites the paramilitaries said they had taken.
A prolonged 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 RSF's integration into the military. The disagreement has delayed the signing an internationally backed agreement with political parties on a transition to democracy.
Civilian forces that signed a draft version of that agreement in December called on Saturday for an immediate halt to hostilities by both the army and the RSF, to stop Sudan sliding towards "the precipice of total collapse".
"This is a pivotal moment in the history of our country," they said in a statement. "This is a war that no one will win, and that will destroy our country forever."
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.
Hemedti, the RSF's commander, has been deputy leader of Sudan's ruling Sovereign Council, headed by Burhan, since 2019.
Doctors said a "very large number" of civilians had been killed and injured in the clashes, which extended into several residential neighbourhoods across the capital. They only confirmed three deaths as people were sheltering from gunfire.
The army said the Sudanese air force was conducting operations against the RSF. Footage from broadcasters showed a military aircraft in the sky above Khartoum, but Reuters could not independently confirm the material.
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.
TV footage showed smoke rising over several areas of Khartoum.
Clashes were also taking place at the headquarters of Sudan's state TV, said an anchor who appeared on screen.
The Sudanese armed forces spokesperson 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.
GUNFIREEyewitnesses reported gunfire in many other parts of the country outside the capital. Those included heavy exchanges of gunfire in Merowe, eyewitnesses told Reuters.
Clashes had also erupted between the RSF and army in the Darfur cities of El Fasher and Nyala, eyewitnesses said.
The RSF said it had taken control of airports in El Fasher as well as in West Darfur state.
International powers - the U.S., Russia, Egypt, Saudi Arabia, the United Nations, the European Union and the African Union - all called for an end to the hostilities.
Chad closed its border with Sudan.
A Saudi Arabian airlines plane at Khartoum airport came under fire during clashes, the state-owned carrier said.
It said it had suspended flights to and from Sudan until further notice. Egypt's national airline, Egyptair, suspended flights to Khartoum for 72 hours.
The army said the RSF had tried to attack its troops in several positions.
The RSF, which analysts say is 100,000 strong, said its forces were attacked first by the army, which had surrounded one of its bases and opened fire with heavy weapons.
Hemedti's RSF evolved from so-called janjaweed militias that fought in a conflict in the 2000s in the Darfur region, where an estimated 2.5 million people were displaced and 300,000 killed.
International Criminal Court prosecutors accused government officials and janjaweed commanders of genocide, war crimes and crimes against humanity in Darfur.
Hemedti had put himself at the forefront of a planned transition towards democracy, unsettling fellow military rulers and triggering a mobilisation of troops in the capital Khartoum.
The rift between the forces came to the surface on Thursday when the army said recent movements by the RSF, particularly in Merowe, were illegal.