Italy has prevented a ferry carrying 1,800 people, mainly Moroccans fleeing the fighting in Libya, from docking in Sicily.
The ship had sailed from Tripoli and asked for permission to refuel on the island after being refused entry to Malta, Italian media said.
Meanwhile, 41 people are feared drowned after a boat carrying migrants capsized off Tunisia, UN officials say.
Five were rescued by another boat heading to the island of Lampedusa.
Since Monday, 21 boats have arrived on the Italian island from Tunisia carrying 1,600 migrants.
The boat which went down at around midnight on Sunday night, three hours after leaving the southern port of Zarzis, had been carrying 41 men and five boys.
"We talked to the five survivors in Lampedusa," UN refugee agency spokesman Federico Fossi told the BBC News website.
"They were in the water for eight hours. They saw one boat passing by but it didn't stop. The other one that rescued them had 45 people on board. That was also in distress and that was rescued by a military ship". BBC News
