US President-elect Donald Trump has been warned against scrapping a historic deal with Iran aimed at preventing the country from developing nuclear weapons.
“I think it would be disastrous. It really would … First of all, for one administration to tear up an agreement that a previous administration made would be unprecedented,” Mr Brennan told the BBC broadcaster.
“I think it would be the height of folly if the next administration were to tear up that agreement,” Brennan added.
Since his election, Trump has been more circumspect, not publicly discussing the international deal with Tehran.
Brennan also expressed hope there would be an improvement in relations between Washington and Moscow.
But he warned: “I think President-elect Trump and the new administration need to be wary of Russian promises. Russian promises in my mind have not given us what it is they had pledged.”
During the presidential campaign, Trump had also said he might authorize the torture of detainees, including waterboarding, to gain information, but has since said he had received advice that made him rethink his approach.