A charity monitoring migrant boats in the Mediterranean said Sunday some of the 47 people on a boat it had earlier warned Italian authorities was in distress off the coast of Libya had drowned.

Alarm Phone, a hotline used by migrants in distress, tweeted that it had first alerted Italian authorities early Saturday morning to the "urgent distress situation" of a boat northwest of Benghazi.

"We are in shock. According to different sources, dozens of people from this boat in distress have drowned," tweeted Alarm Phone Sunday.

AFP was unable to immediately confirm the report.

The statement from Alarm Phone comes exactly two weeks after a shipwreck off the southern Italian coast of Calabria in which at least 76 migrants drowned.

Italy's government is facing a backlash after sharp criticism that it failed to intervene in time to save the migrants.

Earlier Sunday, Alarm Phone tweeted that three merchant vessels were at the scene, but said it did not know if a rescue operation was underway because it had lost contact with the boat.

A previous tweet from Alarm Phone said one of the vessels, the Basilis L, was monitoring the boat while it awaited the arrival of the Libyan coastguard, "to force the people back to Libya where they had tried to escape from".

SeaWatch, a German NGO, tweeted Saturday that its surveillance plane had spotted the migrant boat that was "dangerously overcrowded and in frightening waves".

Nearby was "a merchant vessel that was ordered by the Rescue Coordination Center in Rome to coordinate with the Libyan coastguard," it said.

About two hours later on Saturday, SeaWatch tweeted again. "While the weather is deteriorating and making intervention difficult, Tripoli claims it is unable to send a patrol boat," the tweet read.

"Many times in the past Italian authorities have coordinated a rescue in this area. We ask that the same be done, avoiding new deaths."