The Bush administration allowed the CIA to operate secret prisons on the territory of allied countries in Eastern Europe, Asia and Africa, according to media reports.

During his first week as president, Mr Obama ordered the closure of the black sites, as well as the prison camp at Guantanamo Bay, as part of an overhaul of US detainee policy.

Key issue

In his letter, Mr Panetta also stressed that the CIA no longer employed controversial "harsh interrogation techniques", like "waterboarding", or simulated drowning, which have been widely condemned.

"CIA officers do not tolerate, and will continue to promptly report, any inappropriate behaviour or allegations of abuse," he said.

He also announced that the CIA was no longer allowing outside "contractors" to carry out interrogations.

But the CIA retains the power to detain suspects "on a short-term transitory basis".

The BBC's Kevin Connolly says Mr Panetta's statement has an impressive ring, but the CIA's secret prisons may never have been elaborate affairs in themselves and decommissioning may be straightforward.

The key issue for the Obama administration, our correspondent adds, will be its policy towards suspects who fall into its hands, not the buildings in which they are held.