Up to 150 Europe-bound migrants were missing and feared drowned on Thursday after the boats they were traveling in capsized in the Mediterranean Sea off Libya, the coastguard and U.N. refugee agency said.

Ayoub Gassim, a spokesman for Libya's coastguard, said that two boats carrying around 300 migrants capsized east of the capital, Tripoli. Around 137 migrants were rescued and returned to Libya, he said, and the coastguard has recovered just one body so far.

Charlie Yaxley, a spokesman for the U.N. refugee agency, said they estimate "that 150 migrants are potentially missing and died at sea."

After the uprising that toppled and killed Muammar Gaddafi in 2011, Libya became a major conduit for African migrants and refugees seeking a better life in Europe. Traffickers and armed groups have exploited Libya's chaos since his overthrow, and have been implicated in widespread abuses against migrants, including torture and abduction for ransom.

Earlier this week, the Libyan coastguard intercepted around three dozen migrants off the coast and took them to a detention center near Tripoli where an airstrike killed more than 50 people earlier this month. Over 200 detainees are still being held at the Tajoura detention center, near the front lines of fighting between rival Libyan factions. The U.N. has expressed concern for their safety.