Nearly 700 refugees including dozens of Eritreans have been rescued off Sicily in five operations, as leaders grapple with the issue of illegal immigration at a European Union summit.

Italian coastguard and navy vessels and a Maltese cargo ship have saved asylum-seekers from five boats in the past 24 hours, including at least two that were adrift, officials said on Friday.

"It was a night of rescues in the Strait of Sicily" -- the stretch of water between Sicily and Tunisia, the coastguard said in a statement.

The 318 migrants picked up by the navy were all packed on two boats and were later transferred onto the San March amphibious assault ship taking part in a massive search and rescue operation.

The operation was launched by the Italian government in the wake of an October 3 refugee shipwreck tragedy just off the Italian island of Lampedusa in which 366 asylum seekers perished.

Some of the refugees rescued between Thursday and Friday were also from Eritrea, the country where most of those on the boat that sank came from.

More than 33,000 migrants have landed in Italy so far this year -- nearly three times more than the number for last year. The most common countries of origin are Eritrea and Somalia and now also Syria.

The mayor of Lampedusa, the island where most of the migrants land, warned European leaders of the need for an urgent rethink of immigration policies.

"Unless Europe's approach to asylum and immigration changes, it won't only be the migrants but the EU that drowns off Lampedusa," Giusi Nicolini said in Brussels, where she met with European Parliament head Martin Schulz.

The 300-bed refugee centre on Lampedusa is currently housing 700 and more of those who were rescued are arriving. Italian lawmakers say the squalid conditions in the centre are unacceptable.