Eight suspected human traffickers thought to have forced migrants to stay in the hold of a fishing boat in the Mediterranean as 49 of them suffocated on engine fumes have been arrested on suspicion of homicide, Italian police said.

Some of the traffickers were believed to have kicked the heads of the migrants when they tried to climb out of the hold as the air became unbreathable, prosecutor Michelangelo Patane told a news conference in Catania, Sicily.

The dead migrants were discovered last weekend, packed into a fishing boat also carrying 312 others trying to cross the Mediterranean to Italy from North Africa.

It was the third mass fatality in the Mediterranean this month: last week, up to 50 migrants were unaccounted for when their rubber dinghy sank, a few days after some 200 were presumed dead when their boat capsized off Libya.

Human traffickers are taking advantage of chaos in Libya to charge desperate people thousands of dollars to make the crossing, often in rickety boats packed far beyond their capacity.

Some 2,000 were saved in rescue operations last week alone, and the Italian coastguard said on Tuesday another 116 had been rescued from a sailing boat off Italy's southern tip.

But more than 2,300 have perished this year so far, after almost 3,300 died in 2014, according to the International Organisation for Migration.

The victims and survivors of last weekend's tragedy were brought to Catania on Monday and the suspected traffickers were being held on suspicion of homicide and aiding clandestine immigration.

The town's deputy mayor said the dead would be buried in the local cemetery once post-mortem examinations were finished.