Updated 7.25pm
More than 100 migrants including women and children are reported missing, feared drowned in a large shipwreck off the coast of Libya, UN agencies have reported.
If the numbers are confirmed, it will be the largest loss of life in the central Mediterranean so far this year.
General Ayoub Kacem, a spokesman for the Libyan navy, said that "134 migrants were rescued and a body recovered, while 115 other migrants are still missing".
"A wooden boat carrying around 250 people, including women and children, sank some five nautical miles from the coast according to witness testimony from the migrants who survived," Kacem said in a statement.
Libyan coastguards and local fisherman were involved in rescuing the survivors, Charlie Yaxley, the UNHCR's Global Spokesperson for Africa said.
There are reports they were on board at least one ship that left Al Khoms, a town east of Tripoli on Thursday.
Mr Yaxley said the missing passengers, many from Eritrea, included women and children.
"The status quo cannot continue," he said.
"We cannot ignore that boat journeys are becoming more and more deadly."
The head of the UN refugee agency Filippo Grandi tweeted that it was "the worst Mediterranean tragedy of this year".
"Restoring rescue at sea, ending refugee + migrant detention in Libya, increasing safe pathways out of Libya must happen NOW, before it is too late for many more desperate people," he added.
It comes only a few weeks after some 68 migrants died when an Italy-bound boat sank off Tunisia earlier in July.
The boat, filled with mostly African migrants, tipped over shortly after setting out from the Libyan town of Zuwara, west of Tripoli, with the aim of reaching Italy.
Libya, which has been wracked by chaos since the 2011 uprising that killed president Muammar Gaddafi, has long been a major transit route for migrants, especially from sub-Saharan Africa, desperate to reach Europe.
Humanitarian group SOS Mediterranee said Sunday it has relaunched rescue efforts off Libya seven months after abandoning operations amid a refusal by European ports to accept the migrants.
The Norwegian-flagged Ocean Viking will "conduct search and rescue activities in the central Mediterranean" for SOS Mediterranee and Doctors Without Borders (known by its French initials MSF), the group said in a statement.
It said the exodus from Libya was "one of the most perilous sea crossings in the world".
After nearly three years of operations in which it rescued some 30,000 migrants, the Aquarius had been forced to cease operations because of what the group said was obstruction by some European countries.
Italy, in particular, has become increasingly hardline against accepting undocumented migrants on its territory under a populist government.
In an unrelated rescue, 76 migrants arrived in Malta on Thursday evening.
The Armed Forces of Malta said they intercepted the boat in the Maltese search and rescue zone in the south of the country. They said the men on board were from Ghana, Guinea and south Sudan.