A Sudanese paramilitary force locked in a war with the army has placed new constraints on aid deliveries to territories where it is seeking to cement its control, including areas where famine is spreading, humanitarian workers say.

The move comes as the paramilitary Rapid Support Forces seek to form a parallel government in the west of the country, while it is rapidly losing ground in the capital, Khartoum - developments that could further divide the country, which split from South Sudan in 2011.

It also puts hundreds of thousands of people in the western region of Darfur at greater risk of starvation - many of them displaced in previous rounds of conflict.

Relief workers have previously accused fighters from the RSF of looting aid during more than two years of war still raging in Sudan. They also accuse the army of denying or hindering access to RSF-held areas, worsening hunger and disease.

A dozen aid workers, speaking to Reuters on condition of anonymity, said since late last year the RSF has begun demanding higher fees and oversight of operational processes like recruitment of local staff and security, mirroring practices used by army-aligned authorities and further choking off access. The moves by the RSF, which aid groups are trying to push back against, have not been previously reported.

The war, which erupted out of a power struggle between the army and the RSF, has caused what the United Nations calls the world's largest and most devastating humanitarian crisis.

About half of Sudan's population of 50 million suffers from acute hunger, mostly in territory held or under threat from the RSF. More than 12.5 million people have been displaced.

Aid agencies have failed to provide adequate relief and freezes on USAID funding are expected to add to the challenge.

In December, the Sudan Agency for Relief and Humanitarian Operations (SARHO), which administers aid for the RSF, issued directives, copies of which were seen by Reuters, demanding that humanitarian organisations register via a "cooperation agreement" and set up independent country operations in RSF territory.

Though SARHO agreed last month to suspend the directives until April, aid groups say the restrictions continue.

The tightening of bureaucratic controls is driven partly by the RSF's quest for international legitimacy, but also offers a way to raise funds for a faction facing military setbacks while still controlling swathes of the country including almost all of Darfur, the aid workers said.

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