In an interview with CBS's "Face the Nation" on Sunday, Gates said, "I thought it was a mistake in Libya, and I think it is a mistake in Syria, even if we had intervened more significantly in Syria a year ago or six months ago. We overestimate our ability to determine outcomes.”

"Caution, particularly in terms of arming these groups and in terms of US military involvement, is in order," he said.

"Anybody who says, 'It's going to be clean. It's going to be neat. You can establish safe zones, and it'll be just swell,' well, most wars aren't that way," he said.

Gates, who served under both George W Bush and President Barack Obama, was US defense secretary in 2011 when the United States joined a NATO-led air operation in Libya that helped rebels topple Libyan leader Moamer Kadhafi.

On May 1st, senior US administration officials announced that the US will directly arm the militants to topple Syrian President Bashar al-Assad’s government.

The administration is also considering supplying nonlethal military equipment such as armor, communications gear, night-vision goggles and vehicles to Syrian insurgent groups, officials said.