Sudan's army said on Thursday that it would prolong a ceasefire by 72 hours, but violence continued to rock the capital Khartoum and western Darfur region and there was no confirmation of a truce extension by the paramilitary force it was fighting.

Hundreds have died and tens of thousands fled for their lives in two weeks of conflict between the army and rival Rapid Support Forces (RSF).

Together, they toppled a civilian government in an October 2021 coup but are now locked in a power struggle that has derailed an internationally backed transition to democracy and is threatening to destabilise a fragile region.

The army on Wednesday said it agreed to a new three-day ceasefire through Sunday proposed by a regional African group following one mediated by the U.S. and Saudi Arabia due to expire on Thursday night. On Thursday, the military reiterated it would extend the truce and said it would honour it unilaterally.

There was no immediate word from the RSF.

The army claims it controls most of Sudan's regions and is defeating a large RSF deployment in Khartoum where some residential areas have turned into war zones.

Despite a partial lull in fighting since the first 72-hour ceasefire started, air strikes and anti-aircraft fire could be heard on Thursday in the capital and the nearby cities of Omdurman and Bahri, witnesses and Reuters journalists said.

The White House on Thursday said it was concerned by the ceasefire violations, adding that the situation could worsen at any moment and urging U.S. citizens to leave within 24 to 48 hours.

Fighting has spread to the vast Darfur region, where conflict has simmered ever since civil war erupted two decades ago.

The Darfur Bar Association, a rights group, said at least 52 people had died in attacks by well-armed "militias" on residential neighbourhoods in the city of El Geneina, as well as its main hospital, main market, government buildings and several shelters for internally displaced people.

Militiamen from nomadic Arab tribes entered El Geneina as the fighting between the RSF and army created a security vacuum in recent days, said one resident, who asked to withhold his name due to fear of retribution. They were met with armed members of the Masalit tribe, with clashes extending across the city, causing a new wave of displacement.

El Geneina, Sudan's western-most city, has been the site of repeated tribal conflicts in recent years, leading to people being pushed out of their homes multiple times.

"In the past, it would be in one neighbourhood and the authorities would get involved," the resident said. "But because of what's going on, there's been no intervention."