El-Fasher, Sudan - A major city in Sudan's western region of Darfur has been rocked by fighting involving "heavy weaponry," a senior UN official said Saturday.

Violence erupted in populated areas of El-Fasher, putting about 800,000 people at risk, Clementine Nkweta-Salami, the UN's humanitarian coordinator for Sudan, said in a statement.

Wounded civilians were being rushed to hospital and civilians were trying to flee the fighting, she added.

"I am gravely concerned by the eruption of clashes in (El-Fasher) despite repeated calls to parties to the conflict to refrain from attacking the city," said Nkweta-Salami.

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"I am equally disturbed by reports of the use of heavy weaponry and attacks in highly populated areas in the city center and the outskirts of (El-Fasher), resulting in multiple casualties," she added.

For more than a year, Sudan has suffered a war between the army, headed by the country's de facto leader Abdel Fattah al-Burhan, and the paramilitary Rapid Support Forces (RSF), commanded by his former deputy Mohamed Hamdan Daglo.

The war has killed tens of thousands of people and forced more than 8.5 million to flee their homes in what the United Nations has called the "largest displacement crisis in the world." Hunger and starvation have also gripped the country.

The RSF has seized four out of five state capitals in Darfur, a region about the size of France and home to around one quarter of Sudan's 48 million people.

El-Fasher is the last major city in Darfur that is not under paramilitary control, and the United States warned last month of a looming offensive on the city.

UN chief Antonio Guterres said Saturday he was "very concerned about the ongoing war in Sudan."

"We need an urgent ceasefire and a coordinated international effort to deliver a political process that can get the country back on track," he said in a post on social media site X.