Another 140 have been rescued and returned to Libya, the UNHCR said.
UNHCR Libya tweeted: “Tragic news of a deadly shipwreck off the coasts of Al Khoms.
“Initial reports indicate that over 100 persons may have lost their lives while other 140 have been rescued & disembarked, receiving medical & humanitarian assistance by UNHCR partner IMC.”
The ship left Libya from Al Khoms and there were around 300 people travelling.
The survivors were picked up by local fishermen and then returned to to the coast by Libyan coastguard.
If the total is confirmed, the number of dead would be the highest for a shipwreck in the Mediterranean this year.
Many migrants from Libya attempt to reach Europe in unseaworthy boats.
UNHCR and other UN agencies have called for survivors not to be returned to the conflict zone.
This is a breaking story. More to follow.