Washington DC - President Joe Biden on Tuesday called for Sudan's warring parties to "re-engage" in peace talks and facilitate humanitarian access, in some of his most direct comments yet on the country's deadly conflict.

"I call on the belligerents responsible for Sudanese suffering – the Sudanese Armed Forces (SAF) and the Rapid Support Forces (RSF) – to pull back their forces, facilitate unhindered humanitarian access, and re-engage in negotiations to end this war," Biden said in a statement.

"Let it be clear: the United States will not abandon our commitment to the people of Sudan who deserve freedom, peace, and justice," he added.

Sudan's war began in April of last year and has already killed tens of thousands of people, with some estimates as high as 150,000, according to US envoy Tom Perriello.

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In August, US mediators launched a new bid to broker an end to fighting – but while RSF leaders attended in Switzerland, the SAF were unhappy with the format and did not join, though they were in telephone contact with the mediators.

Biden, whose term will end in January, has pushed with his administration for higher-stakes diplomacy on the Sudan conflict in recent months, hoping to make progress on the devastating war which has descended into a humanitarian crisis.

The bloody conflict pits the RSF, led by Mohamed Hamdan Daglo, against the army headed by General Abdel Fattah al-Burhan.

In his statement, Biden cited particular concern over the city of El-Fasher, one of five state capitals in Sudan's western Darfur region, which has been pounded by the RSF since May.

"The city of El-Fasher, Darfur – home to nearly two million people and hundreds of thousands of displaced persons – has been under a months-long siege by the Rapid Support Forces. That siege has become a full-on assault in recent days," Biden said.

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The United Nations says Sudan faces the world's largest internal displacement crisis, with more than 10 million forced to flee internally or abroad.

Both sides have been accused of war crimes, including deliberately targeting civilians.