War-torn Sudan's capital experienced a communications blackout for several hours on Friday, residents said, as the army and paramilitary forces waged intense battles across Khartoum.

"Vital internet and mobile phone connections which have been critical to sourcing information and supplies during nearly three months of war were out of service as "violent clashes" raged in several parts of the city," witnesses told AFP via landline.

The source of the malfunction was not immediately clear, and some mobile networks were restored by 11:00 am (0900 GMT), according to residents.

Through the morning, columns of black smoke were seen rising near army headquarters in the centre of Khartoum as well as in the city's south.

Witnesses in North Khartoum said there were "clashes using all kinds of weapons".

In Omdurman, just across the Nile River, witnesses reported fighter jets and drones flying overhead.

Since April 15, army chief Abdel Fattah al-Burhan has been at war with his former deputy, Mohamed Hamdan Daglo, who commands the paramilitary Rapid Support Forces.

According to the United Nations, more than 1.5 million Khartoum residents have been forced to flee the air strikes from above, tanks and fighters on the streets and rampant looting.

Millions have remained trapped at home for fear of getting caught in the crossfire of the brutal urban warfare in Khartoum's densely populated neighbourhoods or of being attacked on the street.

They have often relied on the internet to source basic needs, setting up crowdsourcing initiatives for escape routes, food and medicine.

The fighting has killed at least 3,000 people across Sudan, according to the Armed Conflict Location and Event Data Project, with the worst fighting taking place in Khartoum and the western region of Darfur.

On Thursday, the International Criminal Court said it has commenced investigations into alleged war crimes, after increased reports of atrocities, particularly in Darfur, including of sexual violence and civilians being targeted for their ethnicity.