At least 22 people were killed when Sudan's paramilitary Rapid Support Forces (RSF) attacked the city of al-Fashir, a pro-democracy activist group said on Saturday, the worst toll after weeks of stalemate on that front in the country's civil war.

The al-Fashir Resistance Committees said on Facebook that the RSF had fired artillery shells on markets, hospitals and residential apartments, and had used a drone to target a hospital.

The city is the national army's last remaining position in the Darfur region, and a key front in the war with the RSF that has turned Sudan into the world's worst humanitarian crisis.

The activist group said it had counted 22 bodies and the casualty toll was expected to rise.

There was no immediate comment from the RSF, which has in the past denied shelling civilian targets.

More than 300,000 people have fled their homes in al-Fashir as a result of fighting that began in April, the United Nations has said.