More than 100 migrants are missing and feared drowned after a wooden boat capsized off the coast of Libya.
Up to 115 people are missing and 134 were rescued, Libyan navy spokesman Ayoub Qassem said on Thursday.
The UN's refugee agency said earlier that up to 150 people are feared drowned.
The ship left Libya from Al Khoms, a town 120 kilometres east of the capital, Tripoli. There were around 250 people travelling on board, according to the first available accounts from survivors, but it was unclear if one or two vessels were involved.
Filippo Grandi, the UN high commissioner for refugee described it as the "worst Mediterranean tragedy of this year".
He also reiterated his call for migrants rescued at sea not to be returned to Libya where they are kept in detention centres and made even more vulnerable to the volatile political situation in the north African country.
But the Libyan coast guard returned at least 84 migrants to the Tajoura detention centre last night, according to a tweet from a UNHCR spokesperson.
This is the same detention centre that suffered an airstrike earlier this month that killed at least 50 people and injured 130.
UN Secretary-General António Guterres said that he was "horrified" by the reports.
"We need safe, legal routes for migrants and refugees," he tweeted.
Earlier in the week, the UNHCR and International Organisation for Migration (IOM) had met with European states to discuss the situation in the Mediterranean and called for preventing loss of life in Libya.
"The status quo, where search and rescue operations are often left to NGO or commercial vessels, cannot continue," the UN refugee agency's statement read after the meeting.
Libya is a hub for migrants departing for Europe — up to 90% of people crossing the Mediterranean depart from the country.
The UNHCR estimates that 34,196 people have risked their lives trying to reach Europe by sea in 2019 so far.