A pregnant woman fleeing Libya gave birth to a baby boy Wednesday onboard the charity ship that rescued her and dozens of others in the central Mediterranean, the charity said.

"Despite the joyful event, her conditions are now critical, and she needs immediate specialised medical care," said Doctors Without Borders (MSF), which operates the Geo Barents ship.

"For this reason, we are requesting the Maltese and Italian authorities to arrange an urgent medical evacuation for her and for her four sons," with the older three also travelling with her, it said.

They were among 90 people rescued on Tuesday morning from what MSF called "an overcrowded and unstable rubber boat", which it said had left Libya the night before.

Another pregnant woman was also on board, and remains on the Geo Barents, among a total of 255 migrants rescued by MSF in the previous days.

"The difficult journey, the harsh conditions, the extreme stress and the deep dehydration triggered Fatima's labour," the charity said, using a pseudonym for the woman.

The boy was born seven hours later.

The Geo Barents was at the centre of a political row last month after being refused entry into an Italian port except to disembark its most vulnerable migrants, with the others ordered to go elsewhere.

They were finally allowed to disembark in Sicily, following intense criticism from NGOs.

But it was the first sign of a new approach under Prime Minister Giorgia Meloni, who took office in October vowing to reduce the number of migrants arriving on its shores from North Africa.

The central Mediterranean is the deadliest migration route in the world, with more than 17,000 deaths and disappearances recorded by the International Organization for Migration (IOM) since 2014.

Despite the risks, more than 96,800 migrants have disembarked on Italian shores so far this year, according to interior ministry figures.

The vast majority are rescued by the Italian coastguard but Rome has strongly criticised the operation of the charity rescue ships.

Another charity vessel, the Ocean Viking, was taken in by France after being similarly refused a safe harbour in Italy, sparking a diplomatic row between Rome and Paris.