An operation to evacuate thousands of Egyptians and other foreigners trapped in Tripoli is likely to begin within days, probably by sea, the International Organization for Migration (IOM) said on Friday. "IOM is working on evacuating growing numbers of migrants in the Libyan capital who are trapped because of the fighting on the western front, who are increasingly vulnerable and now want to leave," IOM spokeswoman Jemini Pandya told a news briefing. "We have a very limited window of opportunity to carry out this operation because of the fighting." A Libyan rebel advance has cut off supply lines from Tunisia to Tripoli, the stronghold of Muammar Gaddafi's government, and the option of evacuation by road. More than 600,000 foreigners have fled Libya, out of an estimated 1.5 million to 2.5 million foreign residents. Most are migrant workers from elsewhere in Africa and Asia. Thousands of Egyptians and other foreigners, including some journalists stranded in Tripoli, had asked to be evacuated, Pandya said. "We have to address the highly complex logistical, political and security challenges. It is clear that this is going to be a difficult as well as a special operation," she said. "We are looking at all options available, but it will probably have to be by sea." The IOM has chartered another ship in rebel-held Benghazi to evacuate foreigners from the rebel-held western port city of Misrata, she said. The vessel was leaving Misrata on Friday on its 15th mission, with another to follow next week. The IOM has evacuated 8,300 foreigners and wounded Libyan civilians from Misrata.