More than 90 migrants are believed missing after their boat sank off the coast of western Libya, a coastguard spokesman said.

Ayoub Qassem said coastguards had rescued 29 migrants some 26 miles off the shore east of Tripoli, and that survivors said 126 people had been on the rubber boat before one of the sides was ripped and it started taking on water.

Libya is the main departure point for mostly African migrants seeking to cross the Mediterranean to Europe. Smugglers arrange ill-equipped and overcrowded vessels that frequently break down or sink.

Qassem said the boat that sank on Wednesday had left at dawn from Garabulli, about 50km east of Tripoli.

Because of overcrowding one of the sides of the boat got torn and water leaked in

"Because of overcrowding one of the sides of the boat got torn and water leaked in," he said. "Ninety-seven illegal migrants are still missing or they have drowned."

The rate of recorded deaths in the Mediterranean has risen sharply this year, with more than 3,740 migrants drowning on their way to Europe. That nearly matches the toll for the whole of 2015.

The route between Libya and Italy has become the busiest crossing point after a deal between Turkey and the European Union in March largely closed off pathways to Greece.

Smugglers in Libya act with impunity, taking advantage of a security vacuum that developed amid the political chaos following the country's 2011 uprising.

They often send migrants with just enough fuel to reach international waters and be picked up by international rescue vessels.

On Tuesday aid group Doctors Without Borders found 25 dead migrants submerged in water and fuel at the bottom of another rubber boat off Libya.