At least 20 migrants have died after falling into the sea about 50km (30 miles) off the Libyan coast, says the Italian coastguard.
The overcrowded boat was carrying some 500 migrants and a rescue operation is under way, reports say.
“There’s a critical situation… About 200 people fell into the water,” a spokesman told AFP news agency.
The central Mediterranean route for illegal migration to Europe is currently the busiest.
More than 50,000 migrants have reached Italy this year.
The route is also the most deadly.