Up to 150 Europe-bound migrants, including women and children, were missing and feared drowned after the boats they were travelling in capsized in the Mediterranean Sea off Libya, the country's coast guard and the UN refugee agency said.
A UN official described the shipwreck as "the worst Mediterranean tragedy" this year.
The International Rescue Committee said the tragedy was a stark reminder of the humanitarian crisis emerging out of Libya and of the urgent need for search and rescue missions to be resumed in the Mediterranean.
Ayoub Gassim, a spokesman for Libya's coast guard, told The Associated Press that two boats carrying around 300 migrants capsized around 120km east of the capital, Tripoli. Around 137 migrants were rescued and returned to Libya, he said, and several bodies have been recovered.
Charlie Yaxley, a spokesman for the UN refugee agency, said 147 had been saved.
"We estimate that 150 migrants are potentially missing and died at sea," he said. "The dead include women and children."
Mr Yaxley added that so far this year, one person has died on the route from Libya to Europe for every six people that reached Europe shores.
"The worst Mediterranean tragedy of this year has just occurred," said UN 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.
Mr 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".
Asked about the latest boat sinking and deaths, UN deputy spokesman Farhan Haq expressed the world body's concerns.
"We've made clear the need for all the countries in the region to work to ensure first and foremost that the lives of those people who have placed themselves in such great risk are protected," he said.
After the uprising that toppled and killed dictator Muammar Gaddafi 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.
Thomas Garofalo, director for Libya at the International Rescue Committee, which responds to global humanitarian crises, said migrants "intercepted at sea must not be returned to Libya".
