The tragedy happened two weeks after the Cutro disaster that saw at least 79 migrants die on the Calabrian coast.
Alarm Phone tweeted: "We are in shock.
“According to different sources, dozens of people from this boat in distress have drowned. We first alerted authorities at 2:28h CET on 11 March, emphasising the urgent distress situation. The Italian authorities knowingly delayed rescue and let them die."
There has not been official confirmation of the shipwreck.
Many of the 47 migrants aboard a boat drifting off Libya on Sunday are missing after ending up in the water after the boat capsized, the Mediterranea Saving Humans NGO said.
Mediterranea said "a few days after the Cutro disaster we are faced with a new, dramatic case of non-assistance and delay in rescuing people in danger at sea.
"Every effort must be made to locate the missing and save their lives".
Some 79 people are officially dead in the Cutro disaster with a further 18-20 still missing.
ANSA
