Coast guard and other military vessels, aided by aircraft including from the European Frontex mission, were searching a vast area on Saturday.
According to Algerian port authorities, the vessel was believed to have set sail from Algeria two days earlier.
The Italian news agency ANSA said an overturned wooden boat was spotted 40 kilometers (25 miles) off Sardinia's coast.
But Italy's coast guard said there was no immediate confirmation that the vessel was the boat they were searching for.
Cargo ships were also alerted to be on the lookout for migrants or the boat.
A fishing boat was spotted by a Coast Guard ATR 42 flight off Maltese coasts on Saturday, carrying about 300 migrants.
A cargo ship was diverted to the area in order to rescue the boat.
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