Expressing concern over series of arrests of American nationals as terror suspects, US Secretary of State, Hillary Clinton has said much of the training and direction for terrorists comes from Pakistan. Talking to reporters in Washington she said the U.S. has to work more closely with Afghanistan and Pakistan to root out the infrastructure of terrorism that continues to recruit and train. David Headley, a US national of Pakistani origin was arrested by the FBI in October on charges of planning terrorist attacks in Mumbai. Meanwhile, Headley has pleaded not guilty before a US court to all the charges filed against him. He was charged by the FBI on Monday in a 12-count criminal information with six counts of conspiracy to bomb places in India and Denmark and for providing material support to terrorist plots.