Italian rescuers say at least 19 people have died trying to reach Italy on an overcrowded migrant boat.

Officials said Saturday that the migrants apparently died from toxic engine fumes while crammed together in the hold of the boat.

The boat was carrying hundreds of people. Eighteen of the migrants were found dead in the hold. Three men were rushed to the hospital in the Sicilian capital of Palermo, but one died en route.

The United Nations refugee agency estimates that more than 500 migrants have died in the Mediterranean Sea so far this year.