Alarm Phone, a group that picks up calls from migrant vessels in distress, said on Friday it had lost contact with the boat on Wednesday morning.

At the time, the boat was adrift, with no working engine, on high seas about 320km north of the Libyan port of Benghazi and more than 400km from Malta or Italy's southern island of Sicily.

Italian NGO Emergency said on Thursday its Life Support ship and the Ocean Viking, another charity vessel, had unsuccessfully looked for the missing boat for 24 hours but found no sign of any shipwreck.

A representative for Emergency said on Friday the search was ongoing, adding the migrants might have been picked up by another boat or might have managed to fix their engine and continue sailing towards Sicily.

The Italian Coast Guard reported on Thursday the rescue of 423 and 671 migrants in two separate operations in Italian search and rescue waters, and Alarm Phone said they were unrelated to the missing boat.

The Italian coast guard had no immediate comment.

In a separate incident, German charity SOS Humanity said 27 migrants were picked up at sea by an oil tanker and illegally taken back to Libya.

Under international humanitarian law, migrants cannot be forcibly returned to countries where they risk serious ill-treatment and widespread migrant abuse has been extensively documented in Libya.

Performance Shipping, the Greek company that owns the P Long Beach tanker allegedly involved in the incident, did not immediately respond to a request for comment sent via the company's website.

European governments have taken an increasingly hard line on migration, including in Italy, which is facing a surge in sea arrivals.

More than 47,000 landings have been recorded in the year to date, up from about 18,000 in the same period of 2022.

Australian Associated Press