At least 239 migrants are believed to have drowned this week in two shipwrecks off the coast of Libya, the United Nations refugee agency said Thursday, adding to the toll in what was already the deadliest year on record in the Mediterranean Sea.

Survivor accounts suggest that two crowded boats broke up just off the Libyan coast Wednesday, said Carlotta Sami, spokeswoman for the U.N. High Commissioner for Refugees. The 31 survivors were taken Thursday to the Italian island of Lampedusa, which has become a rescue hub amid an ever-deadlier crisis as migrants depart Africa's northern shores trying to reach Europe.

The reports from the survivors could not be independently confirmed, but it is common for migrant ships to be filled far beyond capacity, and hundreds have perished in past sinkings. If true, the latest shipwrecks bring the toll of dead and missing in the Mediterranean to 4,220 this year, the highest on record, Sami said.

"This is an absolutely appalling figure," she said.

According to Sami, the 29 survivors of the first wreck said they capsized after wooden planks at the bottom of the rubber dinghy broke apart several hours after departing Libya around 3 a.m. Wednesday. Pregnant women and at least six children were on board, survivors told the UNHCR, but no children were saved in the rescue, which took place about 25 miles off Libya's coast. One woman lost her 2-month-old baby, Sami said, and 12 bodies were recovered.

The survivors said they were in the cold waters for hours before being rescued about 3 p.m. Wednesday. They said more than 140 people were aboard the boat.

Two survivors of a second shipwreck were rescued in a separate operation, Sami said. They said at least 120 had been on board their boat, which had problems immediately upon setting out and broke apart off the Libyan coast around 5 a.m. Wednesday.

The remaining passengers are believed to have drowned, Sami said. No further rescue operations are being performed at the location of those shipwrecks.

"I am deeply saddened by another tragedy on the high seas. . . . So many lives could be saved through more resettlement and legal pathways to protection," U.N. High Commissioner for Refugees Filippo Grandi said in a statement Thursday. "The Mediterranean is a deadly stretch of sea for refugees and migrants, yet they still see no other option but to risk their lives to cross it."

Most of the migrants appear to have come from sub-Saharan Africa, Sami said, but she said details were still being checked. She did not immediately know which agency carried out the rescue.

The European Union is conducting a search-and-rescue operation in the western Mediterranean that is temporarily being offered logistical help from the NATO military alliance.

"In this, the deadliest year for boat migration to Europe, the E.U. remains focused on deterrence over protection," Judith Sunderland, associate Europe and Central Asia director at Human Rights Watch, said in a statement Thursday. "The EU should be pressing Libyan authorities for permission to operate in Libyan waters, so they can help those in distress and bring them safely to Europe."

Rescued migrants have told the UNHCR that smugglers along the route were telling migrants that responsibility for rescues would soon shift to Libya, and that any rescued refugees would be returned to Libya rather than carried onward to Italy, the agency said. That could be a cause of the current spike.

Migrant traffic across the Mediterranean has changed significantly in the past year, after more than 1 million people made the passage in 2015. Most of them came via Turkey to Greece and then pressed onward into Europe. The sea portion of that journey was shorter and safer than the perilous passage from Libya to Italy. But the Turkish government largely shut down the migrant flow in the spring, closing off the main pathway for people fleeing the conflicts in Syria and Iraq into Europe.

This week, the Gambian soccer federation announced that one of its stars had died at sea while trying to reach Europe. Fatim Jawara, 19, the goalkeeper on the country's women's national team, drowned when her boat went down off the coast of Libya several weeks ago.

Traffic from Libya and northern Africa has increased and grown deadlier, according to U.N. figures. Last year, 153,846 people arrived in Italy via the central Mediterranean route - a figure that has just been surpassed in 2016. The arrivals in Italy last month were more than triple those of a year earlier.

The shifting migration patterns have been a boon to smugglers, as demand has increased across the trickier North African route. Smugglers are sending out large groups in several ships at once, complicating rescue efforts if multiple boats capsize, UNHCR spokesman William Spindler said in October.

It was not immediately clear whether Wednesday's sinkings were connected to a single smuggling operation.