The United Nations said on Thursday it was planning for an outflow of 860,000 people from conflict-torn Sudan, adding that $445 million would be needed to support them just through October.
The UN refugee agency said in a statement that it had presented its appeal to donor countries earlier in the day, and the funds would go to people fleeing into Chad, South Sudan, Egypt, Ethiopia and the Central African Republic.
More than 100,000 people have already fled Sudan since deadly urban combat broke out on April 15 between Sudan's de facto leader Abdel Fattah al-Burhan, who commands the regular army, and his deputy turned rival Mohamed Hamdan Dagalo, who heads the paramilitary Rapid Support Forces (RSF).
Another 330,000 people have been displaced inside Sudan, according to UN figures - and the situation is getting worse.
Thursday's announcement came as gunfire and explosions gripped Khartoum for the 20th straight day, leaving the latest ceasefire effort in tatters.
Even before the fighting erupted, most humanitarian operations in countries neighbouring Sudan were already severely underfunded, with less than 15 percent of funding needs for 2023 covered on average, according to UNHCR.
"We urgently need timely, new funding to respond to the mounting needs," Raouf Mazou, the UN agency's assistance chief of operations, said in a statement.
"The needs are vast, and the challenges are numerous. If the crisis continues, peace and stability across the region could be at stake."
UNHCR said its response plan for the swelling refugee crisis was drawn up with 134 partners, including other UN agencies and national and international non-governmental organisations.
The 860,000 planning figures do not mean the UN necessarily expects this number of people to flee, but that it believes it is possible and is laying financial and operational plans to meet the massive needs that could arise.
Of the total, UNHCR said it was planning for some 580,000 Sudanese people to flee their country, and for 235,000 refugees previously hosted in Sudan to return to their home countries.
Another 45,000 other refugees hosted by Sudan could head to other countries, it said.
According to the plan, Egypt and South Sudan are expected to see the most arrivals.
"The humanitarian situation in and around Sudan is tragic -- there are food, water and fuel shortages, limited access to transport, communications and electricity, and skyrocketing prices of basic items," Mazou said.
"UNHCR and partners have emergency teams in place and are assisting authorities with technical support, registering arrivals, carrying out protection monitoring and strengthening reception to ensure urgent needs are met," he said.
"This is just a start. More help is urgently needed."