Dozens of migrants were feared dead after their boat ran out of fuel in the Mediterranean Sea off the western Libyan town of Zuwara, the coastguard said.

The coastguard found seven survivors and four dead migrants on the boat, which had been floating adrift for several days, spokesman Ayoub Qassem said.

"Another 21 migrants swam to the beach and were later found. It's believed that more than 100 have gone missing," Qassem said.

One of the survivors later died after being taken to hospital for treatment, he said.

Libya is the main departure point for migrants trying to reach Europe by sea. Some 600,000 have crossed to Italy since 2014, though since July there has been a sharp fall in the number of boats attempting the journey.

Migrants are usually packed into flimsy rubber boats with barely enough fuel to get them to international waters, where they hope to be rescued by ships and taken to Italy.

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