More than 30 migrants, mostly toddlers, drowned on Wednesday when about 200 people without life jackets fell from a boat into the sea off the Libyan coast before they could be hauled into waiting rescue boats.
Rescue group MOAS, which operates in the Mediterranean, said its staff was pulling bodies out of the water. "Most are toddlers," co-founder Chris Catrambone said on Twitter.
A total of 34 dead bodies were found in the water, and around 1,800 people rescued from four rubber dinghies and six wooden boats, the coast guard said later in a statement.
British and Spanish navy ships, aid group Doctors Without Borders (MSF), three merchant ships and a tug boat joined MOAS and the Italian Coast Guard and Navy to carry out the rescues.
The ill-fated boat probably tipped because of a combination of weather conditions and the fact the migrants suddenly crowded to one side, sending just under half of the 500 on board into the water, the coast guard said.
More than 1,300 people have died this year on the world's most dangerous crossing for migrants, after boarding flimsy boats to flee poverty and war across Africa and the Middle East.
Last Friday, more than 150 disappeared at sea, the International Organization for Migration (IOM) said Tuesday, citing testimony collected after survivors disembarked in Italy.
In the past week, more than 7,000 migrants have been plucked from boats in international waters off the western coast of Libya, where people smugglers operate with impunity.
Despite efforts by Italy and the European Union to train and equip the U.N.-backed government in Tripoli and its coast guard to fight traffickers, migrants are arriving in record numbers.
Disputes are also brewing between the Libyan Coast Guard and aid groups. MSF and SOS Mediteranee said officials from the Tripoli-based force had boarded a migrant boat during a rescue Tuesday, robbing the migrants and firing shots into the air.
More than 60 people fell into the water in the ensuing panic, but no one was injured as life jackets had already been given out, MSF and SOS Mediteranee said, broadly corroborating an earlier report by humanitarian group Jugend Rettet.
"Italian and European authorities should not be providing support to the Libyan Coast Guard," MSF representative Annemarie Loof said. "This support is further endangering people's lives."