The paramilitaries said they were in control of the presidential place as well as Khartoum airport, claims denied by the army, as civilian leaders called for an immediate ceasefire to prevent the country's "total collapse".

The doctors' union said three civilians had been killed, including at Khartoum airport and in North Kordofan state, and at least nine others wounded.

The eruption of violence came after weeks of deepening tensions between military leader Abdel Fattah al-Burhan and his number two, paramilitary commander Mohamed Hamdan Daglo, over the planned integration of the paramilitary Rapid Support Forces (RSF) into the regular army.

The army said it had carried out air strikes against RSF bases in Khartoum. "The Sudanese air force destroyed Tiba and Soba camps," it said in a statement.

Military leader Burhan has been at loggerheads with his number two, the RSF commander, over talks to finalise a deal to return the country to civilian rule and end the crisis sparked by their 2021 coup.

The RSF said its forces had taken control of Khartoum airport, after witnesses reported seeing truckloads of fighters entering the airport compound, as well as the presidential palace and other key sites.

Its claims were quickly denied by the army, who said the airport and other bases remain under their "full control", publishing a photograph of black smoke billowing from what it said was the RSF headquarters.

The army also accused the paramilitaries of burning civilian airliners at the airport, and Saudi flag carrier Saudia said it had suspended all flights to and from Sudan until further notice after one of its Airbus A330 planes "was involved in an accident".

RSF chief Daglo vowed no let-up. "We will not stop fighting until we capture all the army bases and the honourable members of the armed forces join us," he told Al Jazeera.

- 'Sweeping attack' -

Created in 2013, the RSF emerged from the Janjaweed militia that then president Omar al-Bashir unleashed against non-Arab ethnic minorities in the western Darfur region a decade earlier, drawing accusations of war crimes.

A plan to integrate the RSF into the regular army is one of the key points of contention, analysts have said.

Eleventh-hour haggling between the two men has twice forced postponement of the signing of an agreement with civilian factions setting out a roadmap for the transition.

Witnesses also reported clashes around the state media building in Khartoum's sister city Omdurman, as well near Burhan's residence and in Khartoum North.

Outside the capital, witness Eissa Adam said explosions and gunfire had been heard across the North Darfur state capital of El Fasher, where civilians were hunkered down inside their homes.

The two sides traded blame for starting the fighting.

The RSF said they were "surprised Saturday with a large force from the army entering camps", reporting a "sweeping attack with all kinds of heavy and light weapons".

Army spokesman Brigadier General Nabil Abdallah said the paramilitaries launched the fighting, attacking "several army camps in Khartoum and elsewhere around Sudan".

"Clashes are ongoing and the army is carrying out its duty to safeguard the country", he added.

- 'Slipping into abyss' -

The military's civilian interlocutors called on both sides "to immediately cease hostilities and spare the country slipping into the abyss of total collapse."

Their plea was echoed by US ambassador John Godfrey, who tweeted that he "woke up to the deeply disturbing sounds of gunfire and fighting" and was "currently sheltering in place with the embassy team, as Sudanese throughout Khartoum and elsewhere are doing".

"Escalation of tensions within the military component to direct fighting is extremely dangerous. I urgently call on senior military leaders to stop the fighting," he said.

The head of the United Nations mission in Sudan Volker Perthes called for an "immediate" ceasefire.

"Perthes has reached out to both parties asking them for an immediate cessation of fighting to ensure the safety of the Sudanese people and to spare the country from further violence," the UN mission said.

Western governments had been warning of the dangers of all-out fighting between the rival security forces since the army issued its warning to the paramilitaries on Thursday.

In recent months, Daglo has said the 2021 coup was a "mistake" that failed to bring about change in Sudan and reinvigorated remnants of Bashir's regime, which was ousted by the army in 2019 following month of mass protests.

Burhan, a career soldier from northern Sudan who rose the ranks under Bashir's three-decade rule, maintained that the coup was "necessary" to bring more groups into the political process.