A shipwreck off the Libyan coast has killed 20 people, Doctors Without Borders (MSF) said, just hours after news of a separate incident in which at least 74 migrants died. 

MSF teams in northwestern coastal city of Sorman "assisted three women as the lone survivors of another shipwreck where 20 people drowned," the group said Thursday on Twitter.

"Rescued by local fishermen, they were in shock and terrified; they saw loved ones disappear beneath the waves, dying in front of their eyes," it added.

Earlier Thursday, the UN's International Organization for Migration reported "a devastating shipwreck which claimed the lives of at least 74 migrants today off the coast of Khoms," a port city 120 kilometres (75 miles) east of the Libyan capital Tripoli.

It said that 47 survivors had been brought back to shore and 31 bodies retrieved, adding that the boat was reported to be carrying more than 120 people.

The IOM said that so far this year, at least 900 people had drowned in the Mediterranean trying to reach European shores -- some due to delays in rescue.

More than 11,000 others have been returned to Libya, it said, "putting them at risk of facing human rights violations, detention, abuse, trafficking and exploitation".