A shipwreck off Libya has left at least 15 migrants dead, the UN's migration agency said Wednesday, the latest tragedy to strike people making desperate bids to reach Europe.

"At least 15 migrants are presumed dead after their boat capsized off the coast of Sabratha, #Libya yesterday, according to five survivors brought to shore by fishermen," the International Organization for Migration's Libya chief wrote in a tweet.

"More than 70 others were intercepted and returned to Libya by the coast guard last night," Federico Soda added.

Libya has been ravaged by violence since a NATO-backed uprising toppled and killed longtime dictator Moamer Kadhafi in 2011.

The North African country has become a key embarkation point for migrants attempting death-defying voyages to Europe across the Mediterranean.

So far this year, some 10,000 people have been intercepted at sea and returned to Libya by its coastguard, according to the IOM.

It deems the country unsafe for migrants, who in many cases have been tortured or been sold into slavery.

Amnesty International last month urged the European Union to reconsider cooperation with Libya over "horrific abuses" of refugees and migrants.