CAIRO (AP) — 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 coast guard and the U.N. refugee agency said.

A top U.N. official described the shipwreck as "the worst Mediterranean tragedy" so far this year.

Ayoub Gassim, a spokesman for Libya's coast guard, told The Associated Press that two boats carrying around 300 migrants capsized around 120 kilometers (75 miles) east of the capital, Tripoli. Around 137 migrants were rescued and returned to Libya, he said, and the coast guard has recovered just one body so far.

Charlie Yaxley, a spokesman for the U.N. refugee agency, said 147 had been saved.

"We estimate that 150 migrants are potentially missing and died at sea," he said.

"The worst Mediterranean tragedy of this year has just occurred," said U.N. High Commissioner for Refugees Filippo Grandi.

In January, some 117 died or went missing off Libya's coast and around 65 people drowned after their boat sank off the coast of Tunisia in May.

Grandi called on European nations to resume rescue missions in the Mediterranean, halted after an EU decision, and appealed for an end to migrant detentions in Libya.

He said safe pathways out of the North African country are needed "before it is too late for many more desperate people."

After the uprising that toppled and killed Moammar Gadhafi 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 of migrants, including torture and abduction for ransom.

Earlier this week, the Libyan coast guard 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.

In recent years the European Union has partnered with the coast guard and other Libyan forces to prevent migrants from making the dangerous journey by sea to Europe. Rights groups say those efforts have left migrants at the mercy of brutal armed groups or confined in squalid detention centers that lack adequate food and water.

At least 2,500 migrants are detained in centers in and around Tripoli, where forces loyal to Field Marshal Khalifa Hifter have been battling an array of militias loosely aligned with a U.N.-recognized government since April. The government blamed the airstrike on the detention center on Hifter's forces, which denied responsibility and accused government-linked militias of storing weapons at the facility.

The U.N. refugee agency says 164 migrants have died traveling from Libya to Europe since the start of the year, fewer than in previous years. But the U.N. says the journey is becoming more dangerous for those who attempt it, with one out of four perishing at sea before reaching Europe.

The U.N.'s death toll did not include those reported missing at sea Thursday.