More than 213,000 foreign workers have fled Libya's violence, aid officials said as they sought at least £102 million in emergency help for the exodus.
Hundreds of thousands more people are expected to follow over the next three months.
As part of the emergency appeal to be formally announced later by the UN, the International Organisation for Migration, an inter-governmental body, is appealing for at least £31 million for 65,000 migrant workers scrambling to leave the North African country, spokeswoman Jemini Pandya said.
"This still only scratches the surface," Ms Pandya said. "We have to make sure that the response to the appeal is fast and furious, because time is of the essence."
UN Secretary-General Ban Ki-moon said he is deeply concerned about the plight of the many migrant workers and other civilians, particularly in the western portion that includes Libyan leader Muammar Gaddafi's stronghold in the capital Tripoli.
Mr Ban has appointed former Jordanian foreign minister Abdelilah Al-Khatib as his special envoy to Libya and urged authorities to ensure the safety of all foreigners and to provide unhindered access for humanitarian aid.
He also called for an immediate halt to what he called Gaddafi's "disproportionate use of force and indiscriminate attacks on civilian targets" and said those who violate international law must be held accountable.
Ms Pandya said there were about 1.5 million foreign migrant workers inside Libya before fighting began, along with thousands of Libyan refugees and asylum seekers - so eventually many more millions might be needed.
"We know more migrant workers will flee," she said. "So far, we have seen more than 213,000 migrant workers leave the country, and they represent only 15% of the foreign population. So there are still a lot of people stuck inside the country."
Most of those fleeing have crossed Libya's borders into Tunisia, Egypt and Niger.