Nearly 400 people were still missing from a shipwreck off the coast of Libya, including many unaccompanied children, Save the Children organization said Tuesday.
The boat carrying 550 people capsized Sunday. The Italian Coast Guard recovered nine bodies and 144 survivors from the site, an official said Monday.
There was no confirmation from Italian authorities of the 400 still missing. Save the Children said it calculated the number from descriptions by the survivors who were brought to the southern Italian city of Reggio Calabria.
If the number is confirmed, it would be the worst single refugee catastrophe in the Mediterranean since October 2013, when 366 migrants drowned off the Italian island Lampedusa, provoking Europe-wide consternation.
"More landings are expected as instability continues in Libya, raising concerns that many more children will brave the perilous journey to find safety in Italy," the aid organization said.
In a separate incident Monday, an Italian tugboat and Icelandic Coast Guard vessel - part of the EU's Triton rescue programme - clashed with people smugglers over the rescue of 250 migrants around 110 kilometres off Libya.
The smugglers "fired several shots" to recover the empty wooden boat from which the migrants had been rescued and transferred to the Icelandic vessel Tyr, according to a statement by Frontex, the EU border protection agency, issued Tuesday.
The Italian tugboat was trying to take the wooden boat in tow when the smugglers raced in on a speedboat and sped away with the empty migrant boat.
It was the second such incident since January.
"This is a sign that smugglers in Libya are running short of boats and are more willing to use weapons to recover those used to transport the migrants," said Fabrice Leggeri, Frontex executive director.
Since Friday alone, the Italian Coast guard has rescued about 8,500 people. Many reception centres in Italy are full and overflowing. Migrants rescued in recent days originated from countries in the Horn of Africa, Subsaharan Africa and West Africa, Frontex said.
Last year, a record 170,000 migrants landed on Italian shores, and an estimated 3,419 drowned along the way, prompting humanitarian groups to label the Mediterranean the world's deadliest sea migration route.
Italy closed down its Mare Nostrum rescue programme in October, claiming it was financially unsustainable, and passed the baton to the EU's Triton mission, a much smaller outfit that still relies on substantial Italian contributions.
Sea migrant inflows into Italy - the main entry point for Europe-bound asylum seekers and refugees - have sharply increased in recent days, with the onset of warmer weather.
The Italian office of the United Nations' Refugee Agency said more than 500 people had drowned in the Mediterranean since the start of the year, a 30-fold increase from the same period of 2014.
It was not clear if that number included the 400 reported missing by Save the Children group.
Libya is the launchpad for most migrant boat trips, and law and order there has collapsed following the NATO-backed ouster of former leader Moamer Gaddafi. Italy has called for greater international efforts to stabilize the country and re-establish control over its sea borders.
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