The Italian navy rescued more than 1,000 migrants in the 24 hours to Friday from boats trying to reach Europe, authorities said, as an immigration crisis that killed hundreds in the last year showed no signs of easing.

Navy helicopters spotted four overcrowded boats struggling to stay afloat south of Sicily on Thursday and ships were sent to save them, the navy said in a statement.

The 823 men, women and children aboard the four vessels were from countries including Egypt, Pakistan, Iraq and Tunisia.

The navy rescued 233 migrants from Eritrea, Nigeria, Somalia, Zambia, Mali and Pakistan in a separate operation and took them to a port near Syracuse on Sicily's eastern coast.

Following an October shipwreck in which 366 Eritrean migrants drowned off the Italian island of Lampedusa, Italy launched a special operation combining ships, helicopters and drones to monitor the Mediterranean Sea.

Italy is a major gateway into Europe for many migrants seeking a better life, and sea arrivals to the country from Northern Africa more than tripled in 2013, fuelled by refugees in Syria's civil war and strife in the Horn of Africa.

Over the past two decades, Italy, Greece and the Mediterranean island of Malta have the brunt of the migrant flows and have urged a coordinated European Union response.