More than 30 migrants, mostly toddlers, drowned today 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.

The boat was near a rescue vessel when it suddenly listed and many migrants tumbled into the Mediterranean, Italian Coast Guard commander Cosimo Nicastro told Reuters. "At least 20 dead bodies were spotted in the water," he said.

The rescue group MOAS, which also had a ship nearby, said it had already recovered more than 30 bodies. "Most are toddlers," the group's co-founder Chris Catrambone said on Twitter.

The coast guard called in more ships to help with the rescue, saying about 1,700 people were packed into about 15 vessels in the area.

The transfer from these overloaded boats is risky because desperate migrants in them sometimes surge to the side nearest a rescue vessel and destabilise their flimsy craft, which then list dangerously or capsize.

More than 1,300 people have died this year on the world's most dangerous crossing for migrants fleeing poverty and war across Africa and the Middle East.

Last Friday, more than 150 disappeared at sea, the International Organization for Migration (IOM) said , citing migrant testimony collected after they disembarked in Italy.

In the past week, more than 7,000 migrants have been plucked from unsafe 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 bolster the Tripoli-based coast guard and funnel training and equipment to fight trafficking to the UN-backed government, record numbers of migrants are coming this year.