ROME -- Italian authorities say they have rescued more than 4,000 would-be migrants at sea over the past four days as the war in Syria and instability in Libya spawn new waves of refugees.

The numbers of migrants reaching Italian shores generally rises as warm weather and calm seas make the Mediterranean Sea crossing from North Africa easier. But the UN refugee agency says the 2014 numbers represent a 300 per cent increase over the same period in 2013.

Maritime officials said 2,922 were picked up March 17-18, and another 1,165 in the two days ending Thursday.

Italy beefed up its air and sea surveillance of the smuggling route after a boat capsized last October off the island of Lampedusa, killing more than 360 people.

Nearly 43,000 migrants arrived by sea in 2013.