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.

Over the course of the war momentum on the battlefield has swung back and forth as both sides draw on local and foreign support, with little sign of a decisive breakthrough.

In recent days, however, the army has swiftly retaken ground in the capital that the RSF occupied at the start of the war, including Khartoum's presidential palace, advances documented by a Reuters journalist.

'IMPOSSIBLE CHOICE'

Aid workers say failure to register with SARHO results in arbitrary delays and rejection of travel permits, but that compliance could lead to expulsion by the army and the Port Sudan-based government that is aligned with it.

This presented aid organisations with an "impossible choice," MSF Secretary General Christopher Lockyear told the UN Security Council earlier this month. "Either way, lifesaving assistance hangs in the balance."

Data compiled by the Sudan INGO forum, which represents non-governmental organisations, the proportion of groups facing delays getting travel permits into RSF territory doubled to 60% in January, from 20-30% last year. That dipped only slightly to 55% in February after SARHO temporarily suspended its directives.

"Engagement with SARHO is becoming increasingly challenging," the UN Office for Coordination of Humanitarian Affairs said in an operational update this month. 

In February the UN's top official in Sudan, Clementine Nkweta-Salami, said SARHO's demands risked "corruption and aid diversion".

Both warring parties deny impeding aid.

In an interview with Reuters, SARHO head Abdelrahman Ismail said the agency was exercising its legal rights and duties.

"International humanitarian law gives us the right to organise this work via flexible, straightforward procedures, and in fact dozens of local organisations and a limited number of international organisations signed on," he said.

Authorities in the army-backed administration in Port Sudan were pressuring international organisations not to deal with SARHO, he added.

'HUNGRY AND SCARED'

Aid workers say the restrictions have had the biggest impact in the famine-stricken areas around the city of al-Fashir, the army's besieged final holdout in Darfur, as well as in nearby Tawila, where tens of thousands have sought refuge.

A global hunger monitor has confirmed famine in three camps for displaced people close to al-Fashir - Zamzam, Abu Shouk, and al-Salam. The RSF has also shelled the camps in recent weeks as it seeks to push the army and its allies out.

"The situation in Zamzam camp is very difficult, we are hungry and scared," said a 37-year-old resident of the camp, Haroun Adam. "We aren't receiving any form of aid, and people are eating leaves because there's no food."  

Aid workers said that in addition to seeking oversight, the RSF was increasing fees for various aid operations including hiring local staff and transport of supplies.

The more engagement there was between the RSF and aid agencies, "the more foothold they have to ask for fees," said one aid worker.

Ismail said accusations of interference and exorbitant fees were falsehoods promoted by the army-backed government, and that SARHO was facilitating access.

Impediments to aid are not new in Sudan, where the government sought to manipulate relief operations for decades, said Kholood Khair, a Sudan analyst and director of Confluence Advisory.

According to aid workers, the RSF, which claims to represent Sudan's historically marginalised peripheries, is following the same playbook.

The approach was a "harbinger for what an RSF government will look like", said Khair.