Two boats carrying 300 migrants from Libya to Europe capsized in the Mediterranean Sea on Thursday, and the country's Coast Guard and the U.N. refugee agency believe as many as half the passengers may have drowned, including women and children.

The boats capsized about 75 miles east of the capital, Tripoli; and the Libyan Coast Guard reported just one body found thus far, Ayoub Gassim, a spokesman, told The Associated Press.

AIRSTRIKE IN LIBYA FLATTENS MIGRANT DETENTION CENTER, KILLING AT LEAST 44

Another 137 migrants were rescued and returned to Libya, which is in the midst of a humanitarian crisis arising from the overthrow and death of dictator Moammar Gadhafi in 2011. Since then, Libya has been beset by political unrest, ongoing fights between rival government factions, and reports of trafficking, torture, abuse, and kidnappings. This has prompted many migrants to make the dangerous journey by sea to Europe in search of refuge.

Charlie Yaxley, a spokesman for the U.N. refugee agency, said 147 people had been saved, but estimated "that 150 migrants are potentially missing and died at sea," and that "the dead include women and children."

Yaxley said the death toll of migrants attempting the trip from Libya to European nations continues to climb; roughly one person dies for every six that reach Europe's shores.

U.N. High Commissioner for Refugees Filippo Grandi called the capsizing episode "the worst Mediterranean tragedy of this year."

Grandi urged European nations to resume rescue missions in the Mediterranean, which halted after an E.U. decision, and he pushed for an end to migrant detentions in Libya, adding that safe pathways out of Libya are needed "before it is too late for many more desperate people."

U.N. Secretary-General António Guterres said he was "horrified" by the potential deaths in the shipwreck, echoing Grandi's sentiments through Twitter:

"We need safe, legal routes for migrants and refugees. Every migrant searching for a better life deserves safety and dignity."

Thomas Garofalo, director for Libya at the International Rescue Committee, an advocacy group that helps with global humanitarian crises, said migrants "intercepted at sea must not be returned to Libya."

In January, 117 migrants died or went missing off Libya's coast. Another 65 people drowned after their boat sank off the coast of Tunisia in May.

The E.U. has allied with the Coast Guard and other Libyan agencies to prevent migrants from making the dangerous journey by sea. But advocacy groups say those efforts have subjected migrants to the brutality of armed groups in control in the North African region, or left them contained in detention centers with poor living conditions.

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The Associated Press contributed to this report.