The number of rescued migrants stranded in the central Mediterranean rose to over 500 on Monday afternoon after the charity rescue ship Ocean Viking picked up another group of 105 from the water.

The vessel, formerly the Aquarius, is operated by SOS Mediterranean and Doctors Without Borders (MSF).

"Minutes after life-jackets were distributed, a rubber tube on the fragile boat burst, causing people to fall into the water. All 105 people are now safely on the Ocean Viking, where the total onboard is now 356," SOS Mediterranee said in a tweet.

Another 159 migrants are on board the converted yacht Open Arms. Most of them were picked up near the Libyan coast on August 1 but a further 39 were rescued Friday in Malta's search and rescue area.

Malta has offered to take those 39 but the ship's operator, the Spanish NGO Proactiva insisted all should be allowed to disembark. Malta has refused.  

The Ocean Viking rescued another 81 migrants off the coast of Libya on Sunday, joining 130 others already aboard the ship. Sunday's rescue was the third in as many days.

The young men, mostly Sudanese, who had left Libya late Saturday in a rubber dinghy, clapped and cheered as the ship came into view. 

The ship has been patrolling international waters some 50 nautical miles off the coast of Tripoli.

"We're the only ones in the area, the Libyan coastguard don't respond to distressed migrant vessels," SOS Mediterranean search and rescue coordinator Nicholas Romaniuk told an AFP reporter on board the rescue ship.

Romaniuk said fair weather conditions would likely encourage more departures from Libyan shores.

A further incentive is that a three-day Muslim holiday, Eid al-Adha, which kicked off on Sunday, may reduce the presence of authorities patrolling Libyan beaches.