CAIRO — About 200 migrants are believed to have drowned off the coast of Libya when their boat capsized, the United Nations refugee agency said Thursday.

The disaster, which the agency said took place on Wednesday night about 15 miles off the Libyan coast, is one of the deadliest this summer as thousands of migrants from Africa and as far away as Syria and the Palestinian territories have made their way to the lawless shores of Libya for a chance to get to a better life in Europe. More than 2,100 others have already died this summer.

Smugglers of migrants profit from transporting refugees to Libya and then sending them into the sea, often jammed into flimsy vessels with many more passengers than the boats can hold. The migrants who board them say their goal is to be rescued by the Italian Coast Guard, which will deposit them in a refugee camp in Europe. Few hold out any hope that their overcrowded vessels will make it all the way to Italy.

But the smugglers who launched the boat that sank on Wednesday appear to have packed it with an extraordinarily precarious number of passengers, even by the desperate standards of the trade. Officials of the refugee agency, the United Nations High Commissioner for Refugees, said in a statement that officials believed the smugglers had squeezed about 600 people into a boat that should have carried 40 or 50 at most.