At least 20 migrants including young children perished when an overladen wooden boat capsized off the Libyan coast on Wednesday, Italian coastguard sources said.
Around 200 people were thrown into the water when the boat overturned and sank, possibly after being swamped by a wave, the sources said.
At least 500 people were aboard the boat, according to the sources.
An Italian coastguard vessel, navy helicopters and a boat from the Maltese charity MOAS were rescuing migrants and recovering bodies from the scene of the shipwreck, while other ships were due to join the operation, said the sources.
MOAS co-founder Chris Catrambone tweeted from the charity's Phoenix rescue vessel that the death-toll from the shipwreck was 31, calling it "a big tragedy".
Catrambone published photos showing white body bags lined up on the rescue ship's deck.
"Approximately 30 bodies are still floating in the Rescue Zone... many are small toddlers," read an earlier tweet by Catrambone.
"Not a scene from a horror movie...Real life tragedy unfolding on Europe's doorstep today!"
Rescuers were "frantically" trying to break open the boat's hold where "hundreds" of migrants were trapped," Catrambone tweeted.
A total of 1,252 migrants have died on gone missing on the perilous crossing from Libya to Italy there this year, the International Organization for Migration said on Tuesday.
The central Mediterranean route between Italy and Libya is considered the world's most dangerous for sea migrants.
