Students hold posters during an anti-nukes rally September 24, 2009 in Washington, DC. (Alex Wong/Getty Images)
Obama went public with the charge in a joint appearance with British Prime Minister Gordon Brown and French President Nicolas Sarkozy at a Group of 20 summit in Pittsburgh, sharpening a standoff with Iran over its disputed nuclear program.
Obama called Iran's activities "a direct challenge" to the nuclear nonproliferation regime and said it was time for Tehran to act immediately to restore the confidence of the international community.
