UNITED NATIONS - United Nations Secretary-General Antonio Guterres is "gravely alarmed" by reports of a full-scale assault on the Sudanese city of al-Fashir by the paramilitary Rapid Support Forces (RSF) and called on its leader to halt the attack immediately, a UN spokesperson said on Saturday.

Guterres warned any further escalation threatens to spread the conflict throughout the country's western Darfur region, the spokesperson said.

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"He calls on Lt General Mohamed Hamdan 'Hemedti' Dagalo to act responsibly and immediately order a halt to the RSF attack," UN spokesperson Stephane Dujarric said in a statement. "It is unconscionable that the warring parties have repeatedly ignored calls for a cessation of hostilities."

War erupted in Sudan between the Sudanese army and the RSF in April last year, triggering the world's largest displacement crisis. UN officials have warned the worsening violence around al-Fashir threatens to unleash more intercommunal strife.

In a resolution adopted in June, the UN Security Council demanded a stop to the siege of al-Fashir - a city of 1.8 million people in Sudan's North Darfur region - by the RSF and an immediate end to fighting in the area.

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The resolution also called for the withdrawal of all fighters who threaten the safety and security of civilians in al-Fashir, the last big city in the vast Darfur region not under RSF control.

In the early 2000s the UN estimates some 300,000 people were killed in Darfur when "Janjaweed" militias - from which the RSF formed - helped the army crush a rebellion by mainly non-Arab groups. Sudanese leaders are wanted by the International Criminal Court for genocide and crimes against humanity.