About 7.1 million people are now displaced inside Sudan, the International Organisation for Migration said, with 3.8 million newly displaced as a result of months of fighting between the Sudanese army and the Rapid Support Forces paramilitary group.
Most internally displaced people are now living in the River Nile, East Darfur, Northern, South Darfur, Sennar and White Nile states, it said.
Khartoum continues to be rocked by violence, with fighting resuming at the weekend. Scores of women and children were killed in an army air strike on the capital on Sunday.
On Monday, RSF commander Gen Mohamed Dagalo claimed he was fighting to “return democracy” to Sudan, labelling army chief Abdel Fattah Al Burhan a criminal.
More than one million have fled the country due to the conflict, primarily to Chad, South Sudan, Egypt and Ethiopia.
The agency said it expects 1.8 million people to flee Sudan by the end of this year and appealed for $1 billion aid amid reports of rising disease and death rates.
“The people of Sudan deserve peace. Any further escalation of violence would further devastate the country and the region,” said Federico Soda, director of IOM's Department of Operations and Emergencies.
At least 5,000 people have been killed in the conflict, according to estimates from war-monitoring groups.
The UN has said humanitarian needs in the war-torn country are at an “all-time high”, with half of the population requiring aid. The price of basic goods has skyrocketed and a large number of medical facilities are out of service.
The IOM said it was “committed” to continuing its work in Sudan but admitted the fighting is hindering its efforts to help the most vulnerable.
“Intensified hostilities, fighting, and other obstacles faced by aid organisations continue to make access to people in need extremely challenging in many areas of the country,” said the agency.
More than half of Khartoum's prewar population has fled the capital since fighting began.
Most of the city is now under the control of the RSF, and local residents face regular power and water cuts as well as army air strikes.