Italy's wealthy North vowed Sunday that it would refuse to accommodate any more migrants as thousands more were rescued in the Mediterranean by a multinational flotilla of ships.

As another frantic weekend of rescues unfolded, at least 4,000 people were plucked to safety from packed fishing boats and rubber dinghies off Libya.

Mass drownings in the Mediterranean have claimed nearly 1,800 lives so far this year.

All of those rescued will be deposited at ports on Sicily or elsewhere in southern Italy in the coming days, lifting this year's total of new arrivals on Italian soil to more than 50,000.

The latest batch sent the migration crisis back to the top of the political agenda with three big northern regions vowing to defy the centre-left government of Prime Minister Matteo Renzi by refusing to house any of the new arrivals.

Lombardy president Roberto Maroni said he would be writing to local mayors and prefects in his region on Monday to warn them not to accept any more "illegal immigrants" allocated by the government

Municipalities that did not tow the line would have their funding from the region cut, he said.

Giovanni Toti, the newly-elected president of Liguria, backed that stance.

"We will not receive any more migrants," he said.

Luca Zaia, the right-wing president of Veneto, said the region that includes Venice was "like a bomb ready to go off. The social tensions are absolutely crazy."

British navy ship HMS Bulwark rescued more than 1,000 migrants Sunday from boats in waters between Italy and Libya. At least 10 pregnant women were saved.

The UN's refugee agency said up to 1,500 people were adrift on 10 dinghies, but there was no confirmation of those figures from Italy's coastguard.

On Saturday nearly 3,500 migrants were rescued from 15 packed boats in waters 45 miles (70 kilometres) off the Libyan coast.

There were no reports of casualties but one Italian navy boat ferrying 475 migrants to Sicily reported that it had seven pregnant women on board.

The latest operations will lift the number of arrivals in Italy since the turn of the year to more than 50,000. The cost and other problems involved in processing them is becoming a hot political issue.

- Breaking point -

According to the International Organisation for Migration, nearly 1,800 migrants have drowned attempting to make the crossing since the start of this year, including some 800 in an April sinking that was the biggest maritime disaster in the Mediterranean since World War II.

European Union governments reacted by sending more boats to patrol the area.

But they have been unable to agree on a longer-term strategy to ease the migration crisis amid divisions over how to combat traffickers and spread asylum seekers fairly across member states.

"We cannot simply deal with the symptoms of this problem; we must go after the route cause and the trafficking gangs behind it who are making money out of human misery," British Defence Secretary Michael Fallon said on a visit to HMS Bulwark.

The international naval effort was a successful "temporary" measure, he said.

"But now it's time for Europe as a whole to work together to agree a solution, before we see a greater loss of life."

The 50,000 figure represents an increase of around 10 percent on the same period last year, which, after a summer surge, ended with an unprecedented total of 170,000 migrants arriving on Italian soil.

The country's reception facilities are at breaking point with nearly 80,000 asylum seekers or recently arrived migrants currently accommodated across the country.

Government attempts to get regions to open new facilities are increasingly running into opposition; mainly from right-wing politicians but also at a grassroots level from communities which don't want refugees housed in their neighbourhoods.

The small Alpine region of Val d'Aosta has refused to take any more, citing a lack of adequate facilities.

If Lombardy, Veneto and Liguria act on their threats to follow suit, the government will have a major problem on its hands at a time when it is also grappling with growing evidence that organised crime has been siphoning off public funds allocated for the accommodation of migrants during their processing.