Sudan's military launched air strikes on a paramilitary force's base near the capital in a bid to reassert control over the country on Sunday as clashes killed at least 25 people and threatened efforts to transition to civilian rule.

At the end of a day of heavy fighting, the army struck a base belonging to the government's paramilitary Rapid Support Forces (RSF) in the city of Omdurman, which adjoins the capital Khartoum, eyewitnesses said late on Saturday.

The Sudanese Doctors' Union earlier reported at least 25 people were killed and 183 wounded in battles that erupted on Saturday between the military and the RSF.

The group said it recorded deaths at Khartoum's airport and Omdurman, as well as west of Khartoum in the cities of Nyala, El Obeid and El Fasher.

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 a holiday was declared in Khartoum state for Sunday, closing schools, banks and government offices.

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.

A Reuters journalist saw cannon and armoured vehicles on the streets and heard heavy weapons fire near the headquarters of both the army and RSF.

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 armed forces said it would not negotiate with the RSF unless the force dissolved. The army told soldiers seconded to the RSF to report to nearby army units, which could deplete RSF ranks if they obey.

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

"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," Hemedti said.

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

POLITICAL AGREEMENT AT RISK

The clashes follow rising tensions 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.

A coalition of civilian groups that signed a draft of that agreement in December called on Saturday 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."

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.

Eyewitnesses reported fighting in many areas outside the capital. Those included heavy exchanges of gunfire in Merowe, eyewitnesses told Reuters.

The RSF shared a video that it said showed Egyptian troops who "surrendered" to them in Merowe. Egypt said the troops were in Sudan for exercises with their Sudanese counterparts.

Hemedti told Sky News Arabia the Egyptians were safe and the RSF would cooperate with Cairo on their return.

The video showed men dressed in army fatigues crouched on the ground and speaking in an Egyptian Arabic dialect. Unconfirmed reports by open-source intelligence analysts said several Egyptian Air Force fighter planes and their pilots were captured by the RSF, along with Sudanese weapons and military vehicles.

Clashes also erupted between the RSF and army in the Darfur cities of El Fasher and Nyala, eyewitnesses said.

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

After a phone call, the Saudi, U.S. and UAE foreign ministers called for a return to the framework agreement on the transition to democracy, the Saudi state news agency reported.

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

Reuters