Italian police say they have arrested 15 Muslim migrants suspected to have thrown 12 Christians from a boat crossing the Mediterranean on Thursday.

The dozen men, from Nigeria and Ghana, are believed to be dead. Survivors reported that other Christians on the boat had been threatened, and had to "forcefully resist" attempts to drown them by forming a human chain. The 15 have now been charged with multiple homicide motivated by religious hatred.

"The motive for the resentment was traced to their faiths," police in the Sicilian capital Palermo said following the incident.

The suspects, from Ivory Coast, Mali and Senegal, were among more than 100 people travelling in a rubber boat from Libya, hoping to reach Italy. They were rescued by an Italian Vessel on Wednesday. According to the International Organisation for Migration (IOM), around 20,000 migrants have reached the Italian coast so far this year, having fled war and deprivation in Africa.

The journey is becoming more and more dangerous, and the number of related deaths has risen almost nine-fold since last year. Around 900 have been killed while trying to cross the Mediterranean so far this year, an increase from 96 between January and April in 2014. Around 400 died earlier this week in a shipwreck off the coast of Libya, and another 41 are missing, presumed drowned, in separate incident.

The number of minors making the journey alone has also soared. Underage arrivals to Italy tripled in 2014 from the previous year.

Traffickers are exploiting those attempting to leave Libya, and are believed to charge some $1,000 for every migrant whom they give a passage. Some also turn violent, threatening coast guards with machine guns to avoid having their boats confiscated. 

In November last year, Pope Francis called for a "united response" to the arrival of migrants in Europe, and urged for immediate action to increase protections.

Speaking before the European Parliament in Strasbourg, the Pope said: "We cannot allow the Mediterranean to become a vast cemetery.

"The absence of mutual support within the European Union runs the risk of encouraging... solutions which fail to take into account the human dignity of immigrants, and thus contribute to slave labour and continuing social tensions."

Italy phased out a dedicated maritime search and rescue operation called "Mare Nostrum" or "Our Sea" late last year, making way for a European Union border control mission.

The EU operation, called Triton, has been criticized by humanitarian groups and Italian authorities as it has a much smaller budget and a narrower remit than Mare Nostrum.

Additional reporting by Reuters