The sounds of airstrikes, anti-aircraft weaponry and artillery could be heard in Khartoum early on Saturday and dark smoke rose over parts of the city, as fighting in Sudan entered a third week.

Fighting between the army and a rival paramilitary force continued despite the announcement of a 72-hour ceasefire extension on Friday, when strikes by air, tanks and artillery rocked Khartoum and the adjacent cities of Bahri and Ombdurman.

Hundreds have been killed and tens of thousands have fled for their lives in a power struggle between the army and the paramilitary Rapid Support Forces (RSF) that erupted into violence on April 15, derailing an internationally-backed transition toward democratic elections.

The fighting has also reawakened a two-decade-old conflict in the western Darfur region where scores have died this week.

The army has been deploying jets or drones on RSF forces in neighborhoods across the capital. Many residents are pinned down by urban warfare with scant food, fuel, water and power.

At least 512 people have been killed and close to 4,200 wounded, according to the United Nations, which believes the real toll is much higher.

More than 75,000 people were internally displaced within Sudan just in the first week of the fighting, according to the UN. Only 16 percent of hospitals were operating as normal in the capital.

The latest ceasefire, brokered by foreign powers, is supposed to last until Sunday midnight.

The RSF accused the army of violating it with airstrikes on its bases in Omdurman, Khartoum's sister city at the confluence of the Blue Nile and White Nile rivers, and Mount Awliya.

The army blamed the RSF for violations.

The violence has sent tens of thousands of refugees across Sudan's borders and threatens to stir instability across a volatile swathe of Africa between the Sahel and the Red Sea.

In Darfur, at least 96 people had died since Monday in inter-communal violence rekindled by the army-RSF conflict, UN human rights office spokesperson Ravina Shamdasani said.