The White House said 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.
US President Barack Obama has tip-toed over the 'red-line' and armed Syrian rebels but it is a gamble of huge proportions.
Writing for ITV News, international affairs analyst David Butter looks at the timing of US support for Syria and when it may arrive.
Are the US supplied assault rifles too little, too late and what incentive is there for Assad to enter peace talks?