Addressing a Rome political conference by telephone, Berlusconi said 25 thousand people have arrived on the island of Lampedusa in south Italy since a popular uprising prompted Tunisia's authoritarian president to resign in January.
"Tunisia is not collaborating," Berlusconi said.
Berlusconi said that 10,000 people escaped from Tunisia's prisons amid national protests against Zine El Abidine Ben Ali.
"We suspect, and in some cases are certain, that they have arrived here," Berlusconi added.
"We are talking about 5,000 undesirable Tunisian citizens," he said, without quoting any official source for the information.
Members of the Italian government had already raised the possibility that the migrant boats could be used as a Trojan Horse by Islamic terrorists to enter Europe.
Critics have accused Rome of alarmism and insensitivity toward the plight of North Africans. Supporters of the Italian government say the Tunisians are seeking economic opportunity, not political asylum.
Berlusconi's government has called the arrival of mostly Tunisian migrants a crisis and called on the European Union to relieve Italy by providing funds and taking some of the illegal immigrants off its hands.
Berlusconi visited the tiny southern island of Lampedusa on Wednesday and promised tax breaks to its residents. The locals have accused the central government of abandoning to the thousands of unauthorised visitors who long ago filled Lampedusas' sole detention centre far beyond capacity and have been sleeping outdoors.
At the time of Berlusconi's visit there were around 6,300 migrants crowded on the island - more than its resident population of some 5,000 people.
Around 2,500 people have since been transferred to other sites in Italy, according to SkyNews24.
Italian interior minister Roberto Maroni said the immigrant boats will continue to arrive until Tunisia moves to apprehend the vessels.
It will be resolved "only if and when Tunisia blocks its shores and takes back the illegal immigrants," he said on Thursday.
In the past six weeks, over 19,000 Tunisian migrants have reached Italian shores, have been identified and should be deported, Maroni said.