A humanitarian group said it recovered the bodies of four migrants and rescued around 400 survivors on Tuesday from an overcrowded wooden boat in the Mediterranean between Italy and Libya.

The dead had suffocated below deck, the Malta-based Migrant Offshore Aid Station (MOAS), whose Topaz Responder rescue ship carried out the operation, said on Twitter.

Emergency, another humanitarian organization, was treating another person who was found on the same boat in a critical condition, MOAS said.

A spokesman for the Italian coast guard, whom rescuers have to contact when they pick up people at sea, said around 500 people had been saved in three rescue missions on Tuesday, all around 20 miles (32 km) from the coast of Libya.

The coast guard spokesman, who could not confirm the death toll, said the Bourbon Argos, run by humanitarian group Doctors Without Borders, had gone to the rescue of a rubber boat.

A third mission, involving a small boat carrying about 20 people, was still in progress, the spokesman said.

Italy's Navy said in a separate statement that one of its helicopters had air-lifted one migrant in respiratory arrest toward the island of Lampedusa, and that two of its ships had been involved in rescues. It was not clear whether they were the same incidents reported by the coast guard.

Italy has been on the front line of Europe's worst migration crisis since World War Two, which is now in its third year.

More than 67,000 seaborne migrants had arrived in Italy by early July, according to the International Organisation for Migration, and 2,499 had died attempting the crossing.