Up to 150 people attempting the perilous sea crossing from Libya to Europe are missing and feared drowned after two migrant boats capsized on Thursday.

If the toll is confirmed, it would be the highest from a shipwreck in the Mediterranean this year.

Ayoub Gassim, a spokesman for Libya’s coastguard, told Associated Press that two boats carrying about 300 people capsized about 75 miles (120km) east of the capital, Tripoli. About 137 people were rescued and returned to Libya, he said. The coastguard had recovered one body so far.

Crossings to Europe usually peak in summer when the seas are calmer. The International Organization for Migration has recorded 37,555 people entering Europe by sea and a further 8,007 by land so far in 2019.

The UN high commissioner for refugees (UNHCR) said the number of confirmed deaths on the Libya to Europe route was 164 since the start of the year, fewer than in previous years. But the UN said the journey was becoming more dangerous, with one in four people dying at sea before reaching Europe.

A huge exodus from the shores of northern Africa to Europe began after the 2011 uprisings that toppled the Libyan dictator Muammar Gaddafi.

European leaders have made efforts to stem the flow of boats across the Mediterranean, including partnering with the Libyan coastguard and other Libyan forces. Rights groups said those policies had left migrants at the mercy of brutal armed groups or confined in squalid detention centres that lacked adequate food and water.

A UNHCR spokesman, Charlie Yaxley, said: “This horrific event highlights once again the dire need for a shift in approach to the Mediterranean situation. Urgent action is needed to save lives at sea and prevent people from getting on these boats in the first place by offering safe, legal alternatives.”

Judith Sunderland, associate Europe and central Asia director at Human Rights Watch, said: “What’s always so horrifying about these relentless tragedies is that they should be preventable.

“Instead of conducting European search and rescue in the Mediterranean, the EU has pulled its forces out. Instead of supporting non-governmental rescue operations, EU countries have thrown every obstacle in their way. Instead of taking responsibility themselves, EU countries have been propping up the Libyan coastguard without the resources or dedication to save lives at sea.”

This week, the Libyan coastguard intercepted about three dozen people off the coast and took them to a detention centre near Tripoli. More than 200 detainees are still being held at the Tajoura detention centre near the frontlines of fighting between rival Libyan factions, where an airstrike killed more than 50 people earlier this month. The UN has expressed concern for their safety.

At least 2,500 migrants are detained in centres in and around Tripoli, where forces loyal to Field Marshal Khalifa Haftar are battling militias loosely aligned with a UN-recognised government since April.