Libyan coastguard vessels intercepted two of the migrant boats, the first an inflatable dinghy that had broken down with 125 people on board off Zawiya, just west of the capital, Tripoli, said Ayoub Qassem, a coastguard spokesman.

The second boat was turned back off Garabulli, east of Tripoli, and had 112 people on board.

One boat with more than 100 migrants on board made it as far as the Aquarius rescue ship about 21 miles from the coast to the west of Tripoli. The ship was preparing to transfer the migrants to Italy.

Rescuers on board the Aquarius said the migrant boat was on the verge of completely sinking when it reached them. More than half the migrants on it were Nigerians, with the rest from other sub-Saharan African countries as well as two Palestinians.

“All but one of the five balloons holding the boat up was either completely deflated or deflating and there were still about 120 people inside the boat, so a very, very precarious condition,” said one of the rescuers, Max Avis.

A Reuters reporter saw one of the Palestinians being thrown into the water by other migrants after trying to convince them that they would be better off being saved by a nearby Libyan coastguard boat than drowning.

Libya is the main departure point for migrants attempting to reach Europe by sea. More than 600,000 migrants have crossed the central Mediterranean to Italy over the past four years as people smugglers took advantage of a security vacuum in Libya. Thousands more perished at sea.

Since last summer the rate of departures dropped sharply after smugglers in the Libyan town of Sabratha struck a deal with the Tripoli government to halt their activities and were then pushed from the town by rival armed groups.

Libya’s EU-backed coastguard has also stepped up interceptions, often cutting migrant boats off before they can reach international vessels that would bring them to Europe.

Two other groups of migrants were being transferred to the Aquarius late on Saturday, one from a merchant ship and the other from another charity vessel.

Meanwhile the coastguard in Zuwara, a former Libyan smuggling hub west of Zawiya, said they had foiled a departure during the night and arrested some migrants whilst others had escaped with smugglers.

The coastguard posted pictures of detained sub-Saharan African migrants sitting in an inflatable rubber boat on the beach in the dark.