More than 5,000 migrants on their way to Europe have been saved from boats in distress in the Mediterranean since Friday, according to EU authorities, as the corpses of 17 migrants have been brought ashore in Sicily aboard an Italian naval vessel.

In some of the most intense Mediterranean traffic of the year, migrants who left Libya in 25 boats were picked up by ships from Italy, Britain, Malta and Belgium, assisted by planes from Iceland and Finland, the EU's border control agency Frontex said.

Naval and merchant vessels involved in rescue operations also came from countries including Germany, Ireland and Denmark.

The 17 corpses found on one of the boats arrived in the Sicilian port of Augusta aboard the Italian navy corvette Fenice. Italian prosecutors are investigating how they died.

Frontex is coordinating an EU rescue mission in the Mediterranean known as Triton, which was stepped up after around 800 migrants drowned off Libya in April in the Mediterranean's most deadly shipwreck in living memory.

Accordingly, the EU recently agreed on a naval mission to target gangs smuggling migrants from Libya, but a broader plan to deal with the influx is in doubt due to a dispute over national quotas for housing asylum seekers.

"This is the biggest wave of migrants we have seen in 2015," Frontex Executive Director Fabrice Leggeri said in a written statement.

"The new vessels that joined operation Triton this week have already saved hundreds of people."

Libya is the hub

Italy has so far borne the brunt of Mediterranean rescue operations. Most of the migrants depart from the coast of Libya, which has descended into anarchy since Western powers backed a 2011 revolt that ousted Muammar Gaddafi. Calm seas are increasingly favouring departures as warm spring weather sets in.

The migrants saved over the weekend are all being disembarked at nine ports on the Italian islands of Lampedusa, Sicily and Sardinia and on its southern mainland regions of Calabria and Puglia.

The latest wave of more than 5,000 arrivals will take the total of those reaching Italy by boat across the Mediterranean this year to more than 40,000, according to estimates by the UN refugee agency.

According to the International Organisation for Migration, around 1,770 men, women and children have died or disappeared making the perilous journey.