Khartoum - Sudan's main paramilitary group, the Rapid Support Forces (RSF), said on Saturday that it had seized the presidential palace, the army chief's residence, and Khartoum international airport in an apparent coup attempt. They claimed to have taken over the airports in the northern city of Merowe and in El-Obeid in the west.

The RSF accused the army of attacking them first and plotting with loyalists of former President Omar Hassan al-Bashir to carry out a coup.

The situation on the ground was unclear. The army said it was fighting the RSF at sites the paramilitaries said they had taken. The army also said it had taken some RSF bases and denied that the RSF had taken Merowe airport.

A major 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 and who should oversee the process. The disagreement has delayed the signing of 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."

Doctors said at least three civilians had been killed. Clashes were also taking place at the headquarters of Sudan's state TV, said an anchor who appeared on screen.

Eyewitnesses reported gunfire in many other parts of the country outside the capital. Those included heavy exchanges of gunfire in Merowe, eyewitnesses told Reuters.

The RSF, which analysts say is 100,000 strong, evolved from so-called janjaweed militias that fought in a conflict in the 2000s in the Darfur region. An estimated 2.5 million people were displaced, and 300,000 killed in the conflict.

Hemedti, the RSF leader, 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. The RSF, which together with the army overthrew Bashir four years ago, began redeploying units in Khartoum and elsewhere amid talks last month on its integration into the military under a transition plan that would lead to new elections.

International powers, including the U.S., Russia, Egypt, Saudi Arabia, the United Nations, and the European Union, all called for an end to the hostilities. The army said the RSF had tried to attack its troops in several positions, while the RSF said its forces were attacked first by the army, saying in a statement earlier on Saturday that the army had surrounded one of its bases and opened fire with heavy weapons.