Former national security adviser Condoleezza Rice told jurors she was stunned to learn that a classified mission to thwart Iran's nuclear weapons ambitions had been leaked to the news media.

That leak is now the subject of a criminal trial in federal court in Alexandria. Prosecutors believe ex-CIA case officer Jeffrey Sterling, of O'Fallon, Missouri, illegally disclosed details of the program to journalist James Risen.

In testimony Thursday, Rice told jurors that the program was one of the most closely held operations during her tenure as national security adviser in the first term of George W. Bush's administration.

The plan called for using a Russian CIA asset nicknamed "Merlin" to funnel deliberately flawed nuclear blueprints to the Iranians.

Sterling denies leaking any information to Risen.