Migrants of African origin trying to flee to Europe, disembark in Sfax from a ship owned by the Tunisian coast guards, after being intercepted by them at sea on August 10, 2023. AFP
The rescues took place southwest of Lampedusa, in the waters between the Italian island and Tunisia, the organisation said.
An earlier statement referred to more than 500 people rescued in 11 separate rescue operations.
But a spokeswoman for SOS Mediterranee told AFP Friday evening: "So far, we have rescued 623 migrants since Thursday morning in 15 rescues."
Among those rescued were 146 unaccompanied minors, five accompanied minors, and two pregnant women, said the organisation, which is based in Marseille, on the French Mediterranean.
Some of those rescued were to be taken to Lampedusa and others to the port of Civitavecchia, northwest of Rome, the group added.
Most of those rescued were from Sudan, which has been caught up in a war between rival generals for the past four months. Others were from Guinea, Ivory Coast, Burkina Faso, Benin and Bangladesh.
The Ocean Viking crew were helped on board the vessel by teams from the International Federation of Red Cross and Red Crescent Societies (IFRC), said the earlier SOS Mediterranee statement.
The vessel was alerted Thursday by the Italian emergency services and given the coordinates of vessels in distress southwest of Lampedusa, the NGO said.
The central Mediterranean Sea is the most perilous maritime crossing in the world for the migrants.
At least 1,848 people have died so far this year attempting the Central Mediterranean route from North Africa to Italy and Malta, according to the United Nations migration agency, the International Organization for Migration.
That is significantly higher than its figure for the whole of 2022, which was 1,417.
In June, one sinking alone in the western Mediterranean cost the lives of at least 82 people, one of the deadliest incidents involving migrants in the area.
In July, the Italian authorities detained the Ocean Viking for 10 days at Civitavecchia, after questioning the vessel's safety standards before finally releasing it.