"We had reports this morning that there are seven bodies of illegal migrants that sank off Khoms (east of Tripoli)...but we dont have any details how many migrants were on board," said Mohamad al-Misrati, a spokesman for the Red Crescent in Tripoli.
Fishermen later discovered 30 more bodies in the same area near Khoms, a town some 100 km (62 miles) east of the capital, he said. Volunteers of the Red Crescent were trying to recover the dead but were lacking boats.
No more details were immediately available.
On Thursday, a vessel packed with migrants hoping to make it from Libya to Italy sank in waters off the Libyan town of Zuwara, killing up to 200 people.
Lawless Libya has turned into a major transit route for migrants fleeing conflict and poverty by boat to Europe.
Smugglers exploit the countrys chaos to bring Syrians into Libya via Egypt or nationals of sub-Saharan countries via Niger, Sudan and Chad.
The migrants pay thousands of dollars for the land and sea passage with smugglers often beating and torturing them to press for more money for the final leg of the trip by sea in unseaworthy vessels, rights groups say.
Libya has asked the European Union for help to train and equip its navy which was largely destroyed during the uprising in 2011 that toppled Muammar Gaddafi.
But all training and cooperation was frozen in 2014 as the European Union boycotted a self-declared government controlling western Libya, which seized the capital Tripoli a year ago by expelling the official premier to the east.
Western and most Arab powers only deal with the eastern-based government, which has no control of western Libya from where boats are launched due to its proximity to the Italian island of Lampedusa.
The number of refugees and migrants crossing the Mediterranean to reach Europe has passed 300,000 this year, up from 219,000 in the whole of 2014, the U.N. refugee agency UNHCR said on Friday.
More than 2,500 people have died making the crossing this year, not including those feared drowned off Libya in the last 24 hours, it said. That compares with 3,500 who died or went missing in the Mediterranean in 2014.