Hundreds of migrants attempting the perilous journey across the Mediterranean were feared to have drowned Wednesday after their overcrowded fishing boat capsized off Libya.
The boat ran into difficulty some 15 nautical miles off Libya and sent out a distress call, which was picked up by the coastguard in Sicily.
Two vessels — the Doctors Without Borders (MSF) ship Dignity One and Irish patrol vessel LE Niamh — were immediately dispatched to the scene, but the boat capsized after the migrants all rushed to one side in anticipation of being rescued, the coast guard said.
"Boat carrying 600 capsized off Libya. So far, 250 rescued, 16 dead. Worried for the others," UNHCR chief spokeswoman Melissa Fleming said on Twitter.
Three other ships have been sent to the area to search for survivors.
Nawal Soufi, an Arabic-speaking Italian based in Sicily who is often contacted by migrants in distress, told AFP she had received a call earlier on Wednesday about a boat in trouble with more than 600 people on board.
Soufi said she was called at 9am (0700 GMT) from someone onboard who said the fishing boat, carrying men, women and children, was in trouble because its engine room was flooded.
Coastguard spokesman Filippo Marini confirmed it was probably the same boat but it was too early to say how many people may have disappeared below the waves.
"The boat overturned and sank quickly because it was made of metal," Federico Fossi, spokesman in Italy for the UNHCR told AFP, adding that "we fear many (people) were stuck inside" when it sank.
More than 2,000 people have died trying to cross the Mediterranean to Europe this year, according to the International Organization for Migration (IOM), while around 188,000 people have been rescued.
The latest shipwreck was set to shift the focus in the immigration debate back to the struggles of those fleeing war zones, after weeks of complaints from France and Britain over the disruption caused by asylum seekers at the Channel Tunnel.
European Commission chief Jean-Claude Juncker told AFP Wednesday that EU governments have a duty to help the flood of migrants arriving in Europe and not cave in to "populist" demands to turn them back.
He said he was disappointed that EU ministers had failed late last month to agree on how to distribute a total of 40,000 mostly Syrian and Eritrean migrants from overstretched Italy and Greece.
They agreed to start relocating a little over 32,000 of the arrivals among the EU\’s 28 members in October, falling around 8,000 short of the target agreed by EU leaders at a summit in June.
"If we don\’t get there on a voluntary basis, we will have to reconsider the Commission\’s proposals," the former premier of Luxembourg said.
Nearly all of the people crossing the Mediterranean so far this year, often in rickety boats and at the mercy of human traffickers, have landed either in Italy (97,000) or Greece (90,500), according to the IOM.
The so-called central Mediterranean route proved by far the deadliest, with just over 1,930 people dying as they tried to cross from Libya to Italy, while only about 60 died trying to reach Greece.
The eastern route is shorter and IOM said that traffickers taking people to Italy tended to use more unseaworthy vessels.