The U.N. special envoy for Sudan said talks had yielded the outline of a potential deal on a return to power-sharing, including restoration of the ousted premier, but it had to be agreed in "days not weeks" before both sides' positions harden.

The United Nations has been coordinating efforts to find a way out of the country's crisis following a coup by the military on Oct. 25 in which top civilian politicians were detained and Prime Minister Abdalla Hamdok was placed under house arrest.

Disclosing the "contours" of a potential deal publicly for the first time, the envoy, Volker Perthes, said these included: the return of Hamdok to office, the release of detainees, the lifting of a state of emergency, as well as adjustments to some transitional institutions and a new technocratic cabinet.

In the latest sign of increasing international pressure for a reversal of the coup, U.N. Secretary-General Antonio Guterres spoke with Sudan's General Abdel Fattah al-Burhan on Thursday, urging the restoration of constitutional order and the transitional process.

Perthes, special representative of Guterres and head of the United Nations' Integrated Transition Mission in Sudan (UNITAMS), indicated that time was of the essence.

"The longer you wait the more difficult it is to implement such an agreement and get the necessary buy in from the street and from the political forces," he said in an interview.

"It will also become more difficult for the military, as pressures to appoint some government, whatever its credibility, will increase. And the positions of both sides would harden. We are speaking of days not of weeks," he added.

"Now the question is, are both sides willing to commit to that. Here we still have at least a few hiccups," Perthes told Reuters.

Sudanese state television said on Thursday evening that Burhan had ordered the release of four of the cabinet ministers detained in the coup.

Burhan has said he acted last week to avert civil war after civilian politicians stoked hostility towards the armed forces. He says he is committed to the civilian democratic transition and elections in July 2023.

Neighbourhood resistance committees, which have led protests since the coup and held demonstrations on Thursday, reject negotiations and have demanded that the military exit politics.

The talks effectively represented "the last chance," for the military to come to a negotiated deal, Perthes said, adding that there appeared to be discussions inside the military on whether or not to take advantage of it.

Releasing detainees and resuming internet access would be steps the military could take to de-escalate the situation and garner more support for a deal, Perthes said.

"In any such situations, there are forces inside and outside who encourage the parties to take a harder course, refuse concessions, and simply sit it out," he said.

On Wednesday, Saudi Arabia and the United Arab Emirates joined the United States and Britain in calling for the restoration of the civilian-led government.