The United Nations on Wednesday said nearly three months of war in Sudan have uprooted more than three million people, and called for the warring sides to face "accountability".
Britain said it was taking action. It announced sanctions on businesses it said were associated with Sudanese military groups on both sides of the conflict.
Fighting has raged in the northeast African country since mid-April, when army chief Abdel Fattah al-Burhan and his former deputy Mohamed Hamdan Daglo, who commands the paramilitary Rapid Support Forces (RSF), turned on each other.
Safa Msehli, a spokeswoman for the UN's International Organization for Migration (IOM) in Geneva, told AFP that more than three million people had fled their homes because of the conflict.
IOM figures showed that more than 2.4 million people were now displaced within Sudan, while nearly 724,000 have escaped across the country's borders, in a continuously increasing stream.
"This is more than a figure, however: these are people who have been uprooted, fleeing for their lives," Msehli said, appealing for an immediate cessation of hostilities.
"We need the international community's sustained support to provide aid and protection to those affected by the conflict," she said.
Khartoum's millions of residents, cornered in the capital often without water or electricity in the searing heat of summer, were again subjected to air raids on Wednesday, witnesses said.
"Planes have been pounding RSF bases since dawn," one resident told AFP.
Dozens of civilians have been killed in air raids, and paramilitaries have established bases in residential areas.
Witnesses also reported artillery fire in Khartoum on Wednesday.
The conflict "risks morphing into an ethnicised, tribalised and ideologised conflict which is much closer to being a full-blown civil war," the United Nations Sudan chief Volker Perthes told reporters in Brussels.
- Broken truces -
Perthes, whom Burhan ordered out of the country on June 9 as "persona non grata", said his deputy was running the UN mission from Port Sudan, while he was moving the main office to Kenya's capital Nairobi.
"Accountability is a much broader concept than only a judicial one," he said.
"It is about whether the Sudanese people would allow the leaders of the warring parties to have any leadership function after this war."
Around 3,000 people have been killed, and the UN has warned of possible crimes against humanity in the western region of Darfur.
Perthes said the situation was causing a "humanitarian catastrophe -- again" in Sudan, and putting pressure on neighbouring Chad, which was taking in many refugees and whose food supply lines through Sudan were cut.
IOM figures show more than 239,000 people have crossed into Chad.
Perthes warned of a "risk that neighbouring states are being dragged into the conflict".
He said the UN backed "linked" diplomatic overtures by the east African regional bloc IGAD and an Egypt-hosted summit of Sudan's neighbours on Thursday.
Many truces have been agreed and quickly broken since the fighting in Sudan erupted.
Previous deals have been brokered by Saudi Arabia and the United States, but now the IGAD bloc has said it would ask the African Union to look into possibly deploying the East Africa Standby Force in Sudan "for the protection of civilians and... humanitarian access".
The force is usually tasked with election observer missions.
Human rights violations including "murder, rape and looting," are fuelling a desire among ordinary Sudanese to see the back of the warring generals, Perthes said.
While he was not directly calling for the International Criminal Court (ICC) to bring the generals to justice, he said: "The ICC of course is watching -- it's not up to me to ask the ICC to take action, but I think they are on it."
The United Kingdom on Wednesday said it was sanctioning firms associated with both the army and RSF "fuelling the devastating conflict in Sudan by providing funding and arms".
Foreign Secretary James Cleverly said the sanctions "are directly targeting those whose actions have destroyed the lives of millions" in an "unjustified" war.
Last month the United States also sanctioned companies linked to both the army and the RSF.