French charities SOS Mediterranean and Medecins sans Frontieres rescued 81 migrants off the coast of Libya on Sunday, who joined 130 others aboard the Ocean Viking.
The young men, mostly Sudanese, who left Libya late on Saturday in a rubber dinghy, clapped and cheered as the ship came into view.
Sunday's rescue was the third in as many days. The ship, jointly operated by the two charities, has been patrolling international waters about 90 kilometres off the coast of Tripoli.
"We're the only ones in the area," SOS Mediterranean search and rescue co-ordinator Nicholas Romaniuk said on board the rescue ship. "The Libyan coastguard don't respond."
The ship Open Arms, operated in the same area by the Spanish charity Proactiva, has 160 migrants, of whom 121 have been on board for 10 days while waiting for reluctant EU states to take them in.
Mr Romaniuk said fair weather would probably encourage more migrants to leave Libya.
And the three-day Eid Al Adha, which began on Sunday, might mean fewer patrols of Libyan beaches.
About two thirds of those aboard the Ocean Viking are Sudanese.
The group rescued on Friday were from West Africa, mainly Senegal and Ivory Coast, who went to Libya for work but were caught up in fighting in the lawless country.
Medecins sans Frontieres, which registers the migrants on board, said on Sunday that four fifths of the latest group were aged between 18 and 34, and 17 per cent were under 18.
The rescues come at a time of tension between Italy and other EU states.
The Italian government has refused to let migrants land on its shores unless its EU partners help to take them in.
The Ocean Viking is registered in Norway, and Italy's far-right Interior Minister Matteo Salvini sent a warning to Oslo last week.
"Italy is not legally bound, nor disposed to take in clandestine, unidentified migrants from on board the Ocean Viking," Mr Salvini wrote.
Open Arms founder Oscar Camps on Sunday made a new appeal for European solidarity.
"Tenth day on board, on a scorching Sunday in August," Mr Camps wrote on Twitter.
"We have 160 reasons to carry on, 160 human beings who have the right to disembark at a safe port. Shame on you, Europe."
Open Arms has asked for three ailing migrants to be allowed to disembark. One has TB, one has pneumonia and the other has cancer.