One of Sudan's main landmarks, a skyscraper that towered over the River Nile and housed the headquarters of a major oil company, has been reduced to a smouldering wreck amid heavy fighting between rival military factions in the capital.

The Greater Nile Petroleum Operating Company head office, a glass-sided tower topped with a coil of metal, was built during an oil boom before South Sudan declared independence in 2011, and was one of Sudan's most costly constructions.

Flames and smoke rose from the building in a financial district of Khartoum, close to the confluence of the Blue and White Niles and to areas fought over by Sudan's army and the paramilitary Rapid Support Forces (RSF).

It is unclear what caused the fire that burned through the tower from Saturday. The RSF accused the army of targeting it along with other important buildings amid efforts to dislodge paramilitary fighters from positions they occupied across the capital early in the conflict.

There was no immediate comment from the army.

The war between the army and the RSF broke out in mid-April when tensions linked to an internationally-backed plan for a political transition boiled over, four years after long-time ruler Omar al-Bashir was overthrown during a popular uprising.

The conflict has caused widespread clashes, looting and shortages of food and medicine in Khartoum and other cities, driving more than 5-million people from their homes.