File photo: Sudanese soldiers from the Rapid Support Forces in the East Nile province, Sudan. AP
The Sudan Doctors Network blamed the paramilitary Rapid Support Forces (RSF) for the shelling, which happened in the town of Sennar. As well as the 21 killed, it said more than 70 people had been wounded in the attack.
The attack was just the latest in a bloody conflict that broke out in April last year between the army and paramilitary forces. It has already killed tens of thousands of people and triggered one of the world's worst humanitarian crises.
Sunday's market shelling came a day after Sudan's foreign ministry rejected a call by independent UN experts for "an independent and impartial force with a mandate to safeguard civilians" to be deployed "without delay".
The UN experts spoke out Friday, saying their fact-finding mission had uncovered "harrowing" violations by both sides, "which may amount to war crimes and crimes against humanity".
But the foreign ministry, which is loyal to the army under General Abdel Fattah al-Burhan, ruled out their proposal in a statement issued late Saturday.
"The Sudanese government rejects in their entirety the recommendations of the UN mission," it said.
It denounced the UN Human Rights Council, which created the fact-finding mission last year, as "a political and illegal body", and called the panel's recommendations "a flagrant violation of their mandate".
'Targeting civilians'
The RSF, led by Mohamed Hamdan Daglo, is fighting the Sudanese army under the country's de facto ruler Burhan.
Saturday's foreign ministry statement -- just hours before the market attack -- accused the RSF of "systematically targeting civilians and civilian institutions".
"The protection of civilians remains an absolute priority for the Sudanese government," it added.
The UN Human Rights Council's role should be "to support the national process, rather than seek to impose a different exterior mechanism", it argued.
The ministry also rejected the experts' call for an arms embargo.
The UN experts' report found that eight million civilians have been forced to flee their homes to other parts of the country, while another two million people have fled to neighbouring countries.
More than 25 million people -- over half the country's population -- face acute food shortages.
"Sudanese are suffering through a perfect storm of crises," said World Health Organization chief Tedros Adhanom Ghebreyesus during a visit to Sudan on Sunday.
He listed "over 500 days of conflict", displacement of populations, famine in some areas, natural disasters such as recent floods caused by dams bursting and disease outbreaks.
"The scale of the emergency is shocking, as is the insufficient action being taken to curtail the conflict and respond to the suffering it is causing."
Speaking from Port Sudan -- where government offices and the United Nations have relocated due to the intense fighting in the capital Khartoum -- he called on the "world to wake up and help Sudan out of the nightmare it is living through".
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