At least three people have been killed after long standing tensions between the Sudanese army and a paramilitary group erupted into open fighting.
Witnesses reported clashes around the presidential palace and army headquarters in the capital Khartoum.
The paramilitary Rapid Support Forces (RSF) said they were control of the presidential palace as well as several airports.
The group said they were responding to a surprise attack from the army on one of their bases.
But Sudan's military leader said "all" strategic military sites were under the control of the army.
"No one was able to enter the Army General Command and all the strategic sites are under control," Abdel Fatah al-Burhan said in a statement.
Sudan's army said it was fighting the RSF in Khartoum, accusing the group of "traitorous plotting" against the country.
The Sudanese Doctors' Trade Union has recorded at least three civilian deaths from the clashes.
Meanwhile, medical sources at a hospital in central Khartoum told CNN on Saturday afternoon that the hospital has received dozens of wounded civilians and military personnel in the last several hours.
The military has been in charge of Sudan since a coup in 2021, which ended a power sharing arrangement formed following the ousting of long-term former President Omar al-Bashir.
Talks have been under way on integrating the RSF into the army as part of the move towards the return of civilian rule, but sources told CNN there are tensions over who would be the senior partner in the merger.
International calls for 'immediate ceasefire'
Amid a volatile and uncertain situation on the ground, there have been widespread calls for calm.
The UN Secretary-General António Guterres strongly condemned the outbreak of fighting.
He called on the leaders of the RSF and Sudanese Armed Forces to "immediately cease hostilities, restore calm and initiate a dialogue to resolve the current crisis. Any further escalation in the fighting will have a devastating impact on civilians and further aggravate the already precarious humanitarian situation in the country."
The African Union also issued a statement urging "the political and military parties to find a fair political solution to the crisis."
Sudan's National Umma Party has made a similar plea. The Islamic political party shared a statement containing a list of points directed at the leaders of both sides, including the "immediate cease-fire at all engagement sites" and the retrieval of both sides to "their previous positions before the clashes."
The US ambassador to Sudan said he was sheltering amid the clashes in the capital.
"I just arrived late last night in Khartoum and woke up to the deeply disturbing sounds of gunfire and fighting," Ambassador John Godfrey said in a tweet.
Meanwhile US Secretary of State Antony Blinken said that the situation in Khartoum is "fragile" when asked by a reporter to comment during a presser in Vietnam.
"The major parties in Khartoum, some weeks ago reached a very important framework agreement on how to proceed with a transition to civilian government. And there's been real progress in trying to move that forward," Blinken said on Saturday.
CNN
