Sudan’s military has launched air strikes on a paramilitary force’s base in a bid to reassert control over the country, following clashes in which scores of combatants and at least 56 civilians were killed.

At the end of a day of heavy fighting, the army struck a base belonging to the paramilitary Rapid Support Forces (RSF) in the city of Omdurman, which adjoins the capital Khartoum.

The military and RSF, which analysts say is 100,000 strong, have been competing for power as political factions negotiate forming a transitional government after a 2021 military coup.

In the early hours of Sunday morning, witnesses heard heavy artillery firing across Khartoum, Omdurman and nearby Bahri, and in the Red Sea city of Port Sudan.

The Sudanese Doctors’ Union reported at least 56 civilians had been killed and 595 people, including combatants, had been wounded since the fighting erupted on Saturday.

Scores of military personnel were also killed it said, without giving a specific number due to a lack of first hand information from hospitals where those casualties were taken.

The RSF claimed to have seized the presidential palace, army chief’s residence, state television station and airports in Khartoum, the northern city of Merowe, El Fasher and West Darfur state. The army rejected those assertions.

The Sudanese air force told people to stay indoors while it conducted what it called an aerial survey of RSF activity, and  schools, banks and government offices in Khartoum were closed.

Gunfire and explosions could be heard across the capital, where TV footage showed smoke rising from several districts and social media videos captured military jets flying low over the city, at least one appearing to fire a missile.

Army chief General Abdel Fattah Al-Burhan told Al Jazeera TV the RSF should back down: “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.”

The RSF leader, General Mohamed Hamdan Dagalo, better known as Hemedti, called Burhan a “criminal” and a “liar”.

The RSF accused the army of carrying out a plot by loyalists of former strongman President Omar Hassan al-Bashir – who was ousted in a coup in 2019 – and attempting a coup itself. The 2021 coup ousted the country’s civilian prime minister.

A prolonged confrontation could plunge Sudan into widespread conflict as it struggles with economic breakdown and tribal violence, derailing efforts to move towards elections.

The power struggle has delayed the signing of an internationally backed agreement for a transition to democracy.

A coalition of civilian groups has called for an immediate halt to hostilities, 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.”

International powers – the United States, Russia, Egypt, Saudi Arabia, the United Nations, European Union and African Union – all appealed for an immediate end to the hostilities.

US Secretary of State Antony Blinken said on Saturday he had consulted with the ‍foreign ministers of Saudi Arabia and United Arab Emirates ‍and they’d agreed it was essential for the involved parties in Sudan to immediately end hostilities without any preconditions.

UN Secretary-General Antonio Guterres spoke with Burhan, Hemedti, and Egyptian President Abdel Fattah al-Sisi, Guterres’ spokesperson said.

By Khalid Abdelaziz and Nafisa Eltahir in Khartoum