At least 21 people have been killed and more than 70 others wounded by shelling at a busy market in south-eastern Sudan, a doctors' union says.

The Sudan Doctors Network said the paramilitary Rapid Support Forces (RSF) were responsible for the attack in the city of Sennar on Sunday, condemning it as a "massacre" of civilians.

It happened a day after Sudan's military rejected a proposal by UN experts to send in an international force to protect civilians.

Thousands of people have been killed and more than 10 million have fled their homes since civil war broke out between the army and the RSF last April, making it one of the world's worst humanitarian crises.

Several rounds of peace talks brokered by Saudi Arabia and the US have failed to end the conflict.

The RSF controls most of the capital, Khartoum, much of Kordofan state and most of Darfur - where it is accused of using rape as a weapon of war, and targeting the Masalit people and other non-Arab communities in a campaign of ethnic cleansing.

It has not succeeded in capturing Sennar city from the army.

In June, it seized most of the wider state of Sennar, which analysts say is strategically important because of its position close to Ethiopia and South Sudan and its rich agricultural production.

Both sides in Sudan's conflict, the army and the RSF, are accused of committing atrocities against civilians.

"Harrowing" discoveries made by the UN in a fact-finding mission "may amount to war crimes and crimes against humanity" by both sides, it said last week.

This was angrily rebuked on Saturday by the foreign ministry which is loyal to army chief Gen Abdel Fattah al-Burhan.

"The Sudanese government rejects in their entirety the recommendations of the UN mission," it said, calling the UN Human Rights Council behind the fact-finding mission "a political and illegal body".

The RSF has not commented.

Led by Mohamed Hamdan "Hemedti" Dagalo, the RSF emerged from the Janjaweed militia which was accused of genocide against non-Arab communities in Darfur in 2003.

In recent months it has sought to boost its international profile and seek legitimacy as a political player by sending delegates to peace talks in Switzerland that were snubbed by the army.