Italy has rescued 896 migrants crossing the Mediterranean by boat in the past 24 hours, the navy said on Friday, after the government warned of an "emergency" caused by a surge in new arrivals.

Three navy warships and two coast guard patrol boats took on the migrants who are now being transferred to other vessels and taken to ports in Sicily for identification, the navy said in a statement.

The anti-immigration Northern League, a small but vociferous opposition party, has called for migrants to be sent back and, along with a series of far-right parties, accuses the government of failing to act.

Interior Minister Angelino Alfano has appealed for more assistance from the European Union but migrant rights campaigners say the real fault is with failures in Italy's immigration and asylum system.

Activists say Italy's immigration centres are full and asylum-seekers are being put up in temporary or private facilities like hotels which are not offering adequate assistance to the new arrivals.