"450 migrants are on board a merchant ship with no crew which is headed towards the coast of Puglia," a Twitter message from the coastguard said.
The new high-seas drama came two days after Italian sailors intercepted a freighter carrying more than 700 mostly Syrian migrants which had been heading for the rocks of Italy's southeastern shore on autopilot, having been abandoned by the people smugglers who had navigated it from Turkey via Greek waters.
In that incident, the Moldovan-registered Blue Sky M cargo ship got to within five miles (eight kilometres) -- 45 minutes sailing time -- of a disaster before six navy officers were lowered on to the ship by helicopter and succeeded in bringing it under control.
Many of the migrants on the ship were treated for hypothermia and broken limbs.
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The migrant boat dramas have come as Italy grapples with the aftermath of the Norman Atlantic ferry disaster in which at least 13 people have died following an onboard fire that erupted before dawn on Sunday in waters off Albania.
They also come after a record year for people fleeing conflict and poverty in the Middle East, Africa and Asia attempting to reach Europe by sea.
They are almost invariably under the control of ruthless traffickers who earn thousands of dollars for every person they put to sea from Libya and other departure points in North Africa.
Increasingly, the traffickers appear to have decided that the best way to get their human cargoes to Europe is to put to sea and then abandon the boats.
Since the onset of winter they have been using bigger boats than the converted fishing boats and dinghies they previously favoured.