Italian police say they have arrested 15 Muslim migrants after they allegedly threw 12 Christians overboard following a row on a boat heading to Italy.
The Christian migrants, said to be from Ghana and Nigeria, are all feared dead.
In a separate incident, more than 40 people drowned after another migrant boat sank between Libya and Italy.
Almost 10,000 migrants trying to cross the Mediterranean have been rescued in recent days. Italy has called for more help from the EU to handle the crisis.
More than 500 people from Africa and the Middle East have died making the perilous crossing since the start of the year. Earlier this week, 400 people were believed to have drowned when their boat capsized.
In the latest sinking, the Italian navy plucked four survivors – a Ghanaian, two Nigerians, and a man from Niger – from the sea and took them to Sicily along with 600 other migrants trying to make the crossing in various vessels.
They told the police their inflatable boat sank not long after leaving the coast of Libya with 45 people on board.
Meanwhile, police in Palermo say that 15 Muslim migrants, who travelled on another boat, were arrested on charges of “multiple aggravated murder motivated by religious hate”, after several surviving migrants came forward and told them of an altercation which resulted in 12 Christians being thrown overboard.
The men who have been charged come from the Ivory Coast, Senegal, Mali and Guinea.
They were among 105 migrants travelling in an inflatable boat that left Libya on Tuesday.
In numbers: Migration from north Africa to Europe
The survivors, also Christians, told police that they had only been saved because they strenuously resisted any attempt to be thrown overboard, which in some cases led to them forming a human chain.
Many of them were in tears when they gave their statement, the police added.
Earlier on Thursday Italian Foreign Minister Paolo Gentiloni said Italy had “not had an adequate response from the EU”.
But the European Union has said it has no “silver bullet” for the problem of thousands of migrants trying to reach Europe.
Last year a record 170,000 people fleeing poverty and conflict in Africa and the Middle East made the perilous crossing to Italy.
With improving weather conditions in recent days, the number of people making the crossing of at least 500km (310 miles) has surged. But vessels provided by people smugglers are often underpowered and overcrowded.
Analysis: Gavin Lee, Europe reporter in Brussels
€2.8m (£2m) a month goes on Operation Triton, the border control policy that operates off the Italian coast. Monitoring the Mediterranean may not be enough, says commission spokesperson Natasha Bertaud. “We have neither the money nor the political support to launch a European border guard system,” she told reporters.
Triton has proved an inadequate replacement for the Italian military search-and-rescue operation Mare Nostrum, which cost three times as much. That 2013 mission was activated after a similar tragedy, when 300 migrants drowned.
The Italian government has requested more financial help from the EU, but the question is: how much money are the 28 member states willing to invest?
Only 22 of the members are supporting the current system. Others, including the UK, opted out, describing the policy as unintentionally encouraging more migrant attempts to make the crossing.
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- Video Italy boat rescue: ‘Keep searching’ 4 October 2013
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source: BBC NEWS