(CNN) -- At least 150 people drowned when a boat leaving Libya capsized off the Tunisian coast this week, the U.N. refugee agency said Friday.
Scores more were still missing as a rescue operation by Tunisian authorities continued. But at least 578 people survived the sinking Wednesday, according to the U.N. High Commissioner for Refugees.
On Thursday, 195 survivors were transferred to a humanitarian aid camp close to the Tunisia-Libya border, according to a statement from the refugee agency.
An additional 383 survivors were expected to be taken to the same or nearby camps for help.
The overcrowded boat, which set sail May 28 from Tripoli, was headed for the Italian island of Lampedusa carrying about 850 people. The passengers were mostly migrants from West Africa, Pakistan and Bangladesh who were fleeing fighting in Libya, the refugee agency said in a statement.
The agency said some survivors reported the boat was manned by people with little or no maritime experience.
Soon after the boat left Tripoli, it ran into difficulties and by Tuesday, the passengers ran out of food and water.
The boat ran aground Wednesday near the Kerkennah Islands, about 300 kilometers (about 186 miles) northwest of Tripoli in the Mediterranean Sea, the refugee agency reported.
"Desperate passengers rushed to one side, seeking rescue by the Tunisian coast guard and fishing boats that had approached the vessel," and the boat capsized, the agency said.
"This appears to be one of the worst and the deadliest incidents in the Mediterranean so far this year," said Adrian Edwards, a spokesman for the refugee agency.
Lampedusa, the closest Italian island to Africa, has become a destination for tens of thousands of refugees seeking to enter Europe.
More than 30,000 migrants and refugees from Tunisia and Libya have risked the dangerous journey to Lampedusa.
At one point, the population of migrants vastly outnumbered the tiny population of Lampedusa, which numbers less than 6,000.
"Boats loaded with migrants fleeing the ongoing conflict in Libya have been making the journey to Italy and Malta over recent months, sometimes with tragic consequences," the refugee agency said in its statement. "Just last month, hundreds died as a vessel carrying about 600 people broke up shortly after departing Tripoli."
