Journalists were shown the wrecked building but it was not clear if there were any casualties.

It comes as a coalition of countries including the US, UK and France continue strikes to enforce a UN-mandated no-fly zone over Libya.

Col Gaddafi has been fighting a rebellion that broke out last month.

US officials said Col Gaddafi himself is not a target of the air strikes, which they say are aimed at his armed forces and air defence systems.

On March 20 night anti-aircraft fire rose over Tripoli and several explosions were heard.

Earlier at the Pentagon, Navy Vice Adm William Gortney said the military action, which began on March 19, had been "very effective in significantly degrading the regime's air defence capability".

He said ground forces moving on rebel positions were also open to attack. But he added that neither Col Gaddafi nor his residence would be targeted.

Col Gaddafi has ruled Libya for more than 40 years. An uprising against him began last month after the long-time leaders of neighbouring Tunisia and Egypt were toppled.