Updated 8pm -
Some 30 migrants were missing and feared dead after their boat capsized during a rescue on Sunday off the Libyan coast, the Italian Coast Guard said.
Seventeen were rescued including two injured persons who are being brought to Malta for hospital treatment.
The Italian Coast Guard in a statement said that the NGO Alarm Phone had reported to the Italian, Maltese and Libyan authorities (on Saturday) that a boat with 47 migrants on board was in distress in the Libyan Search and Rescue Area about 100 miles off the Libyan coast.
The boat was sighted by the maritime patrol aircraft Seabird 2, belonging to a German NGO which issued a distress call and contacted the merchant Basilis L which headed towards the small boat.
All information was also provided to the Libyan and Maltese authorities.
The Basilis L reached the small boat but rescuing those on board was difficult because of the rough sea.
The Libyan authorities, who were responsible for search and rescue activities in that area, requested help because they had no available naval assets.
The Rome rescue centre issued a message for all ships in the area and three merchant ships converged on the spot while the Basilis L also stayed there.
One of the four ships, the Froland, started a rescue operation at dawn on Sunday but the small boat capsized. Seventeen people were rescued while some 30 migrants were missing.
Two of the migrants recovered by the Froland required medical assistance and the ship was therefore directed to Malta to disembark them.
Several ships, two aircraft continuing the search
The search for the missing migrants is continuing by the four ships already on site and another two which joined them, as well as two aircraft from Frontex, the EU border agency.
Earlier, Alarm Phone tweeted that it had first alerted Italian authorities early Saturday morning to the "urgent distress situation" of a boat northwest of Benghazi.
"We are in shock. According to different sources, dozens of people from this boat in distress have drowned," Alarm Phone tweeted on Sunday.
The latest tragedy comes exactly two weeks after a shipwreck off the southern Italian coast of Calabria in which at least 76 migrants drowned.
Italy's government is facing sharp criticism that it failed to intervene in time to save the migrants. Similar criticism was directed at the government on Sunday for having the rescue in the Mediterranean.
Alarm Phone said the Italian, Maltese, and Libyan authorities were informed early on Saturday that the boat was in distress. No action was taken even though a nearby cargo ship could have been directed to the boat.
Nine hours after the first alert, the Seabird 2 spotted the boat and also informing the authorities about the urgency of the situation.
"Still, only merchant vessels – not Italian assets or assets of operation IRINI – reached the scene of distress after many hours. This delay, one of the many systematic delays Alarm Phone has documented over the years, proved to be deadly. For many hours, the merchant vessels were merely monitoring the situation but not intervening. Clearly, the Italian authorities were trying to avoid that the people would be brought to Italy, delaying intervention so that the so-called Libyan coastguards would arrive and forcibly return people to Libya, back to the torturous conditions they had tried to escape from," Alarm Phone said.