More than 250 migrants are believed to have drowned after two partially submerged dinghies were found by a rescue boat.
One woman was found dead while more than 1,000 refugees were rescued off the Libyan coast, aid groups said on Sunday.
Around 400 people were crammed onto a single wooden boat, while others were picked up from huge inflatable dinghies, which had set sail from the coast of Libya, according to Doctors Without Borders (MSF) and SOS Mediterranee.
A young woman was found unconscious on one of the vessels and later died, MSF said.
— MSF Sea (@MSF_Sea) March 26, 2017
Two rescue ships found the refugees, just days after 250 migrants and refugees from African countries were reported missing in boat disasters off the Libyan coast.
The weekend rescues mean that about 22,000 mainly African refugees have been picked up heading to Italy so far this year, while around 520 have died trying to make the crossing.
An Italian prosecutor said last week that humanitarian ships operating off Libya were undermining the fight against people smugglers and opening a corridor that is ultimately leading to more deaths.
Non-governmental organisations deny any wrongdoing, saying they are simply looking to save lives.
Despite rough winter seas, refugee departures from Libya on boats chartered by people traffickers have accelerated in recent months from already record levels.
More people died in the Mediterranean over the first nine weeks of 2017 compared to the same period in 2016, the International Organization for Migration (IOM) says. From January 1 to March 9 this year, at least 521 people drowned while attempting to cross the rough waters compared to 471 in the same period a year ago.
There were about 5,000 recorded deaths in all of 2016.
Last month, European leaders signed a controversial plan to help stem the flow of African migrants to Europe. As part of the deal, the European Union will give $215m to Libya’s fragile government to step up efforts to stop boats in the country’s territorial waters.
The EU will also provide support for the setting up of “safe” camps in Libya and the voluntary repatriation of refugees willing to return to their countries of origin.
The plan has been criticised by several aid groups, however, that say leaders have abandoned humanitarian values and misrepresented conditions in Libya, where the UN-backed government of Fayez Serraj has only shaky and partial hold on the country.