WASHINGTON (Reuters) - U.S. intelligence agencies underestimated Islamic State activity inside Syria, which has become "ground zero" for jihadists worldwide, President Barack Obama said in a CBS television interview broadcast on Sunday. Conversely, the United States overestimated the ability of the Iraqi army to fight the militant groups, Obama said in a "60 Minutes" interview taped on Friday, days after the U.S. president made his case at the United Nations for action. Citing earlier comments by James Clapper, director of national intelligence, Obama ack**wledged that U.S. ...