Italy's coastguard said on Sunday that about 30 migrants were missing and presumed drowned after the overcrowded boat they were on capsized during a rescue attempt by a cargo ship off Libya's coast.
An early morning rescue was launched after 17 migrants were saved in a zone under the jurisdiction of Libya, the coastguard said.
"During the rescue operations … the boat capsized during the transfer of the migrants," the coastguard said.
"Seventeen people were rescued and recovered by the vessel while approximately 30 migrants were missing."
The latest disaster in the Mediterranean comes exactly two weeks after a shipwreck off Italy's southern coast of Calabria killed at least 76 people, with bodies continuing to wash ashore almost daily.
That shipwreck has put Italy's far-right government on the defensive as it tries to defend against criticism that it failed to intervene in time to save the migrants.
Alarm Phone, a charity that monitors migrant boats, said it had been contacted by the boat in distress about 160km north-west of Benghazi and alerted Italian authorities early on Saturday morning.
The boat was also seen by a surveillance plane belonging to German NGO SeaWatch, which reported it was "dangerously overcrowded and in frightening waves".
Italy's coastguard said Alarm Phone had notified Rome's rescue co-ordination centre, as well as Maltese and Libyan authorities about the boat.
A merchant vessel that sailed towards the boat after the alert by SeaWatch reported difficulty in rescuing the it because of bad weather, the coastguard said.
Authorities in Libya, which the coastguard said was responsible for search and rescue in that zone, reported "a lack of availability of naval assets" and requested the support of Rome, which sent three more nearby merchant ships to the area, the coastguard said.
A rescue operation began early on Sunday morning after the ship Froland reached the site, but the boat capsized during the operation, the coastguard said.
Two migrants pulled safely onboard needed urgent medical care and the ship headed to Malta to deliver them.
Merchant ships were still in the area searching for the missing, along with two Frontex planes, it said.
"The rescue operation took place outside the Italian SAR [search-and-rescue] area of responsibility, recording the inactivity of the other National Maritime Co-ordination and Rescue Centres involved in the area," the coastguard said.
SeaWatch tweeted on Saturday that bad weather was making a rescue difficult and that "Tripoli claims it is unable to send a patrol boat".
Earlier on Sunday, Alarm Phone tweeted that it had lost contact with the boat and pleaded with authorities not to allow the Libyan coastguard "to force the people back to Libya where they had tried to escape from".
Italy migrant boat tragedy - in pictures
Also on Sunday, intelligence reports indicate nearly 700,000 migrants are in Libya awaiting an opportunity to set out by sea towards Italy, said a politician from Prime Minister Giorgia Meloni’s far-right party.
But a UN migration official said that the number was not credible.
Tommaso Foti, the lower parliamentary house whip for the Brothers of Italy Party, told TV channel Tgcom24 that the Italian secret services estimated 685,000 migrants in Libya, many of them in detention camps, were eager to sail across the central Mediterranean Sea in smugglers' boats.
Ms Meloni is hoping an EU meeting this month yields concrete solidarity from fellow leaders of EU nations in managing the large numbers of migrants and asylum-seekers who sail to countries on the Mediterranean’s rim, including Greece, Cyprus, Malta and Spain, as well as Italy.
“Europe can’t look the other way,″ Mr Foti said.
While the intelligence services assessment sparked alarming headlines in Italy, a spokesman for the International Organisation for Migration said the figure appeared to be confusing the high end of the estimated number of migrants in Libya with those who were seeking to head from there to Europe.
“This number seems to be an estimate, that we also give, of the total presence in Libya,'' Flavio Di Giacomo told AP in Rome.
But of that number, “only a minimum part want to leave and only a minimum part succeeds in leaving" for Europe, Mr Di Giacomo said.
Many migrants in Libya come from Niger and Chad, two African nations on Libya's southern border, and eventually return to their homelands, he said.
The Italian intelligence service's estimate “is the last of a long series of alarms that we've seen in the last 10, 12 years, that turned out to be mistaken'', Mr Di Giacomo said.
”That number doesn't seem to be absolutely credible."
About 105,000 migrants reached Italy by sea in 2022.
From the start of this year until March 10, about 17,600 arrived, including a few thousand who disembarked at Italian ports in the past several days.
That is about triple the number for the same time period in each of the two previous years, although the Covid-19 pandemic might have led to fewer voyages.
For years, Italy has tried with limited success to induce Libya to stop launches of people smugglers' unseaworthy fishing boats and rubber dinghies towards Italian shores.
Italian governments have trained and equipped the Libyan coastguard.
But the traffickers behind the smuggling rings continue to operate amid Libya's feuding political and militant factions.
The IOM and humanitarian groups say passengers whose vessels are turned back by the Libyan coastguard are often returned to detention camps.
There they are at risk of abuse, including torture, until their families raise enough money for the migrants to set out again by sea.
Ms Meloni's government has made it harder for humanitarian organisations that operate rescue boats to carry out many rescues in the waters off Libya, adopting rules that force the vessels to disembark migrants in northern Italian ports, delaying their return to sea.
But many migrants set out from Libya on smugglers' boats.
"It is a worrisome humanitarian flow because people die at sea,'' Mr Di Giacomo said.
The UN migration agency estimates that about 300 people have died this year, or were missing and presumed dead, after trying to cross the central Mediterranean route.