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.
Italian Red Cross personnel prepare to give first aid to shipwrecked migrants as they arrive in the Italian port of Augusta in Sicily on 16 April 2015.
The 15 Muslim migrants involved in the row with Christians were arrested in the Sicilian city of Palermo and charged with "multiple aggravated murder motivated by religious hate".
The suspects, who are from the Ivory Coast, Senegal, Mali and Guinea, were among 105 migrants traveling in an inflatable boat that left Libya on Tuesday.
Eyewitnesses told police how the altercation resulted in Christians being thrown overboard, and that some of the survivors had formed human chains to avoid a similar fate.