The United States is withholding a visa for Sudanese army chief Abdel Fattah al-Burhan ahead of his scheduled UN appearance in a move that Sudanese and other sources say is linked to a US-backed ceasefire proposal that Khartoum has resisted.

The dispute, first reported by Reuters, threatens to deepen tensions between Washington and Sudan's army at a critical moment in the civil war, which the United Nations calls the world's largest humanitarian crisis.

A senior Sudanese source said Burhan had postponed his trip to New York but had yet to receive official notice that the visa had been denied.

"We in the Sudanese government express our deep regret because this is a violation of the UN Convention and international law," the source said. "Sudan's government affirms its stance towards peace."

Sudan's UN ambassador appealed to Secretary-General Antonio Guterres to intervene, arguing in a letter that preventing Burhan from attending the General Assembly would violate the UN's obligations toward member states.

US officials have made Burhan's attendance contingent on his agreeing to a 90-day ceasefire proposed by US Senior Advisor for Arab and African Affairs Massad Boulos, according to a Sudanese diplomatic source and a Western analyst close to American and Sudanese officials.

The US State Department declined to comment on Burhan's visa.

State Department spokesman Tommy Pigott said in a statement later on Tuesday that neither the Sudanese Armed Forces, the Rapid Support Forces (RSF) paramilitary nor either of their respective leaders "represent legitimate, constitutional governance for Sudan."

"The United States and President Trump stand ready to work with anyone who is a serious partner for peace in Sudan. In the meantime, we will seek to raise the cost of continued conflict on all belligerents and the networks which fuel the conflict," he said.

The impasse represents a nadir in US-Sudan relations since war broke out in Sudan between the army and the RSF in April 2023, and comes as Burhan was scheduled to address the General Assembly on Thursday.

Another Sudanese source told Reuters that foreign minister Mohieldin Salem would give the address instead.

Sudan's army voiced reservations about the Boulos-proposed ceasefire in July. 

Burhan is unlikely to engage with Boulos' efforts further if the visa is not granted, the analyst and diplomatic sources said. 

"Burhan rejects this take-it-or-leave-it approach, any proposal is subject to suggestions and changes," the Sudanese diplomatic source said.

The letter dated September 21, addressed to UN Secretary-General Antonio Guterres and signed by Sudan's UN Ambassador Al-Harith Idriss Al-Harith, asked for the United Nations' "immediate intervention" with US authorities to ensure the issuance of the visa.

"Should this situation persist, it would effectively prevent the Head of State of a Member State of the United Nations from participating in person in the work of the General Assembly," read the letter. 

UN 'DEEPLY CONCERNED'

In a statement sent to Reuters, UN spokesperson Stéphane Dujarric said the secretary-general is "deeply concerned that a visa has reportedly not yet been issued to the President of the Transitional Sovereign Council of the Sudan," adding that he is engaging with the US on the issue.

Burhan, the head of Sudan's army, has been the country's de facto leader since 2019, though the African Union and Western powers withdrew recognition after a 2021 coup he led with RSF head Mohamed Hamdan Dagalo, against the civilian leadership. 

Burhan has spoken at the UN General Assembly on Sudan's behalf since the coup. 

Dagalo, known as Hemedti, has set up a rival government that has so far struggled to gain legitimacy.

The United States has made efforts to mediate the conflict within a so-called "Quad" format that also includes the United Arab Emirates, Egypt and Saudi Arabia, countries seen as having the most influence in Sudan. 

In a piece in US outlet Foreign Policy published on Monday, Burhan said he would refuse external directives about the war and said previous pauses in fighting had only allowed the RSF and its backers to regroup.  

Under the agreement for the United States to host the UN headquarters in New York, it is not supposed to deny visas, though it has denied entry to the head of the Palestinian Authority.

Relations between the US and Sudan's army have already been strained in recent weeks over a US proposal to extend an existing arms embargo over Sudan's Darfur region – the RSF's stronghold – to the rest of the country, a move that Burhan's backers say would disproportionately punish the army. 

For his part, in a speech sending soldiers off to battle, Dagalo said "it's time for the rifles to speak."

Sudan's civilian prime minister before the coup, Abdalla Hamdok, is in New York this week as part of the General Assembly, along with members of his anti-war coalition.