The clashes began at dawn, several residents reported, in what appeared to be the army's first major offensive in months to regain parts of the capital controlled by its rival paramilitary Rapid Support Forces.

It comes with Sudan high on the agenda of the United Nations centrepiece meetings in New York this week.

On the sidelines of the talks, UN Secretary-General Antonio Guterres voiced concern to army chief Abdel-Fattah al-Burhan "about the escalation of the conflict in the Sudan", the UN said Wednesday.

Sudanese army forces were "waging fierce fighting against the rebel militia inside Khartoum", a source in the military told AFP, referring to the RSF.

The source, requesting anonymity because they were not authorised to speak to the media, said army forces had crossed three key bridges over the Nile River -- which had separated parts of the capital held by the army from those under RSF control.

Since April 2023, when war broke out between Burhan's Sudanese Armed Forces and his former deputy, RSF commander Mohamed Hamdan Daglo, the paramilitaries had pushed the army nearly all the way out of Khartoum.

Following its last major offensive in February, the army regained much of Omdurman, the capital's twin city just across the river, and part of greater Khartoum.

- 'Intense' shelling -

Several residents of Omdurman reported "intense artillery shelling" which started early on Thursday, with shells falling on residential buildings while military warplanes flew overhead.

Since the war began, much of its worst fighting has been in densely populated areas, and both sides have been accused of indiscriminately bombing residential areas.

The war has already killed tens of thousands of people, with estimates ranging widely from 20,000 to 150,000, according to medics.

It has also created the world's largest displacement crisis, the UN says.

More than 10 million people -- around a fifth of Sudan's population -- have been forced from their homes inside the country and another two million have fled to neighbouring states, according to the United Nations.

Famine has been declared in Zamzam refugee camp in Darfur near the city of El-Fasher, where the RSF last weekend launched a large-scale offensive after months of siege.

El-Fasher is the only one of five state capitals in the vast Darfur region not yet in RSF hands.

In his meeting with Burhan, Guterres said the war "risks a regional spillover," the UN said.

"People in Sudan have endured 17 months of hell, and the suffering continues to grow," the UN's top relief official Joyce Msuya said separately.