The conflict since April last year, pitting the army against paramilitary forces, has killed tens of thousands of people and triggered one of the world's worst humanitarian crises.
The independent UN experts said Friday their fact-finding mission had uncovered "harrowing" violations by both sides, "which may amount to war crimes and crimes against humanity".
They called for "an independent and impartial force with a mandate to safeguard civilians" to be deployed "without delay".
The Sudanese foreign ministry, which is loyal to the army under General Abdel Fattah al-Burhan, said in a statement late Saturday that "the Sudanese government rejects in their entirety the recommendations of the UN mission".
It called the UN Human Rights Council, which created the fact-finding mission last year, "a political and illegal body", and the panel's recommendations "a flagrant violation of their mandate".
The UN experts said eight million civilians have been internally displaced and another two million people have fled to neighbouring countries.
More than 25 million people -- over half the country's population -- face acute food shortages.
World Health Organization chief Tedros Adhanom Ghebreyesus, on a visit to Sudan on Sunday, said "Sudanese are suffering through a perfect storm of crises".
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 -- the WHO chief called on the "world to wake up and help Sudan out of the nightmare it is living through".
The Sudanese foreign ministry statement on Saturday accused the paramilitary Rapid Support Forces, led by Burhan's former deputy Mohamed Hamdan Daglo, of "systematically targeting civilians and civilian institutions".
"The protection of civilians remains an absolute priority for the Sudanese government," it said.
The statement added that the UN Human Rights Council's role should be "to support the national process, rather than seek to impose a different exterior mechanism".
It also rejected the experts' call for an arms embargo.
