(CNN) -- At least 150 people drowned when a boat leaving Libya capsized off the Tunisian coast on Wednesday, the U.N. refugee agency said Friday.
Scores more were still missing Friday as a rescue operation by Tunisian authorities continued.
The overcrowded boat, which set sail Saturday from Tripoli, was headed for Lampedusa, in Italy, carrying about 850 people. The passengers were mostly migrants from West Africa, Pakistan and Bangladesh who were fleeing fighting in Libya, the UN High Commissioner for Refugees (UNHCR) said in a statement.
The agency, which did not indicate how many people had been rescued, 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, some 300 kilometers (about 186 miles) northwest of Tripoli in the Mediterranean Sea, UNHCR 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 in a statement.
"This appears to be one of the worst and the deadliest incidents in the Mediterranean so far this year," Adrian Edwards, a UNHCR spokesman in Geneva, said.
Lampedusa, the closest Italian island to Africa, has become a destination for tens of thousands of refugees seeking to enter the European Union.
More than 30,000 migrants and refugees from Tunisia and Libya have risked the dangerous journey to Lampedusa since last February.
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. Just last month, hundreds died as a vessel carrying about 600 people broke up shortly after departing Tripoli," UNHCR said in its statement.