At least three life rings were thrown into the water near the man, who was named as Pateh Sabally, a 22-year-old Gambian, but he did not appear to reach for them, raising speculation he wanted to commit suicide. No-one jumped in to help him.

Police said the incident happened on Sunday.

A video recording shown on various news websites appeared to pick up some bystanders yelling at the flailing man. "He is stupid. He wants to die," says one person.

More than 181,000 migrants arrived in Italy by boat in 2016, most of them from sub-Saharan Africa, an increase of almost 18 percent compared with 2015.

Italian media said Sabally had residency papers for Italy.