(Adds details of rescues, updates sourcing)
ROME, Aug 12 (Reuters) - Up to 50 migrants went missing after a large rubber dinghy sank in the Mediterranean Sea, Italian rescuers said on Wednesday, while more than 1,500 were picked up from other vessels in the past 24 hours.
The Mediterranean has become the world's most deadly border zone for migrants. More than 2,000 migrants and refugees have died so far this year in attempts to reach Europe by boat, compared with 3,279 deaths during the whole of last year, the International Organization for Migration said last week.
An Italian navy helicopter on Tuesday spotted a rubber boat that appeared to be deflating, the navy said, and dropped life rafts to the migrants on board. The boat then sank, it said.
The Italian naval ship Mimbelli rushed to the scene and pulled 52 migrants to safety. Survivors said there had been about 100 on board, leaving up to about 50 unaccounted for, a rescue operations source said.
A helicopter later airlifted to safety two migrants seen hanging onto a floating barrel near where the dinghy had sunk, the navy said. The survivors were being taken to the Italian island of Lampedusa.
Overall on Tuesday, Italy's coastguard said it coordinated the rescue of more than 1,500 migrants -- many fleeing war zones and poverty in Africa and the Middle East -- from seven different vessels.
People-smugglers, mostly based in lawless Libya and charging thousands of dollars for passage, have sent more than 100,000 migrants by sea to Italy so far this year, according to an Interior Ministry tally. Italy took in 170,000 in 2014.
Around 200 migrants were presumed killed earlier this month off the coast of Libya when their boat capsized. More than 400 were rescued from that shipwreck.
Financially strapped Greece has also struggled to cope with a surge in migrants and refugees arriving on its Mediterranean shores.
Greek police used fire extinguishers and batons against migrants on the island of Kos on Tuesday after violence broke out in a sports stadium where hundreds of people, including young children, were waiting for immigration papers.
Reporting by Philip Pullella, Steve Scherer and Antonella Cinelli; Editing by Mark Heinrich; Editing by Crispian Balmer