"The area where he is now is under siege," Justice Minister Mohammed al-Alagi told Reuters. "The rebels are monitoring the area and they are dealing with it."
Alagi, a lawyer who said he had come to Tripoli to establish the new "legal authority" declined to specify where Gaddafi was.
Other rebel officials have said they believe the fallen strongman has taken refuge in the Abu Salim area in the south of the capital -- an area that saw clashes in recent days.
Rebel fighters have said earlier this week that they thought they had Gaddafi cornered but these reports have turned out to be inaccurate, or at least premature.