WASHINGTON - The White House said on Friday it would be dramatically more difficult and costly to set up a no-fly zone over Syria than it was in Libya, stressing that the United States does not have a national interest in pursuing that option."We feel like the best course of action is to try to strengthen a moderate opposition," Deputy National Security Advisor Ben Rhodes told a news briefing.He said the United States did not want to send US troops - or "boots on the ground" - to Syria and said enforcing a no-fly zone over the country could require intense, open-ended US military engagement.