Libya's coast guard recovered dozens of bodies of Europe-bound migrants who perished at sea as search operations continued Friday, a day after up to 150 people, including women and children, went missing and were feared drowned after their boats capsized in the Mediterranean Sea.

A top UN official described Thursday's shipwreck as "the worst Mediterranean tragedy" so far this year.

Also Friday, Libyan authorities transferred dozens of migrants rescued from the disaster to a detention centre near Tripoli that was hit by an airstrike earlier this month despite UN objections to such a move, the UN refugee agency said.

The Anti-Illegal Immigration Agency in the capital, Tripoli, said that up to 350 migrants were on board the boats that capsized on Thursday off the coast of the Libyan town of Khoms, around 120 kilometres (75 miles) east of Tripoli.

The migrants include nationals from Eretria, Egypt, Sudan and Libya, the agency said.

Libyan officials said more than 130 migrants have been rescued since Thursday.

At least a dozen were taken to a hospital in Khoms while the rest were transferred to different detention centres, including Tajoura, located near the front lines of the fighting between rival Libyan factions.

The officials spoke on condition of anonymity because they were not authorised to brief media.

Tajoura was hit by an airstrike on July 3 that killed more than 50 people and raised new concerns over the treatment of migrants in Libya.

Charlie Yaxley, a spokesman for the UN refugee agency, on Thursday objected to transfers of rescued migrants to Tajoura, saying, "this has to stop" and that the detention centre should be closed.

"Our joint-call to close Tajoura detention center does not seem to be heard. This is putting intentionally the life of these people at risk," Vincent Cochetel, the refugee agency's special envoy for the Central Mediterranean tweeted on Friday.

After the NATO-backed uprising that toppled and killed longtime ruler 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.

Thursday's shipwreck was "the worst Mediterranean tragedy" so far this year, said 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.

At least 2,500 migrants have been detained in centres in and around Tripoli, where forces loyal to commander Khalifa Hifter are battling an array of militias loosely aligned with a U.N.-recognized government.

Hifter's offensive — an attempt to seize Tripoli — started in April and has killed more than 1,000 people, mainly combatants but also civilians, the U.S. said earlier this month.

The Tripoli-based government has blamed the Tajoura airstrike on Hifter's forces, which have denied responsibility and accuse government-linked militias of storing weapons at the facility.

The UN refugee agency says 164 migrants died travelling from Libya to Europe since the start of the year, fewer than in previous years.

But the agency says the journey is becoming more dangerous for those who attempt it, with one out of four perishing at sea before reaching Europe.

The UN's death toll of 154 did not include those reported missing at sea Thursday.