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, Italy's coastguard said on Sunday.
Seventeen migrants were saved and a search was underway for the missing after the early-morning attempted rescue in a search-and-rescue zone under the jurisdiction of Libya, the coastguard said.
"During the rescue operations... the boat capsized during the transfer of the migrants: 17 people were rescued and recovered by the (cargo) vessel while approximately 30 migrants were missing," said the coastguard.
The latest disaster in the Mediterranean comes exactly two weeks after a shipwreck off Italy's southern coast of Calabria that killed at least 76 migrants, with bodies continuing to wash ashore nearly daily.
That shipwreck has put Italy's right-wing government on the defensive as it tries to fend off sharp criticism that it failed to intervene in time to save the migrants.
Sunday's botched rescue is sure to increase finger-pointing over who is to blame for deaths even as an increasing number of migrants embark on the perilous Central Mediterranean crossing headed for Italy's shores.
Italy's coastguard said the boat in distress was in Libya's search-and-rescue zone, and said the deadly episode laid bare "the inactivity" of other countries in the Mediterranean.
Rome has long complained to its EU partners that it bears the brunt of the tens of thousands of migrants seeking to reach Europe who arrive by boat each year.
First to be alerted to the boat in distress some 100 miles (160 kilometres) northwest of Benghazi was Alarm Phone, a charity that monitors migrant boats. It said it then alerted Italian authorities at 0118 GMT Saturday morning.
The boat was also spotted by a surveillance plane of German NGO SeaWatch, which reported it was "dangerously overcrowded and in frightening waves".
In a statement, Italy's coastguard confirmed that Alarm Phone had notified Rome's rescue coordination centre, as well as Maltese and Libyan authorities, about the boat.
A merchant vessel that had headed towards the boat after the alert by SeaWatch reported difficulty in rescuing the boat due to bad weather, the coastguard said.
Authorities in Libya -- which the coastguard said was responsible for search and rescue efforts in that zone -- reported "a lack of availability of naval assets" and requested the support of Rome, which dispatched three additional nearby merchant ships to the area, according to the statement.
SeaWatch also had tweeted on Saturday that Tripoli said it was "unable to send a patrol boat."
After the cargo vessel "Froland" reached the site, a rescue operation began early Sunday morning during which the migrant boat overturned, the coastguard said.
Two migrants among the 17 pulled safely onboard needed urgent medical care and the ship headed to Malta to disembark 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 Coordination and Rescue Centres involved in the area," the coastguard said.
On Sunday morning, Alarm Phone said it had received another distress call from the migrants, more than 24 hours after the charity had first alerted authorities.
"They're exhausted and still out at sea, fighting the bad wind & weather conditions," Alarm Phone tweeted.
In a subsequent message on Twitter, Alarm Phone said it had lost contact with the boat.