A woman who gave birth at sea was among 93 migrants rescued off Libyan shores as they tried to reach Europe, but six others died along the way, the UN's migration agency said.

The situation of refugees and migrants in Libya worsened after eastern Libya-based military commander Khalifa Haftar launched an assault on Tripoli in 2019 and the onset of the novel coronavirus pandemic.

Human rights groups have repeatedly criticised the systematic return of migrants intercepted in the Mediterranean to Libya, where they are held in crowded detention centres.

The latest operation came just days after French charity vessel Ocean Viking picked up dozens of migrants off the Italian island of Lampedusa after they had drifted from Libya.

Nicholas Romaniuk, who coordinated the mission aboard that vessel, said rescue ships are often out-run by the Libyan coastguard who beat them to intercept migrants and return them to Libya.