The head of the US military's special operations confirmed Friday that the Central Intelligence Agency is shutting down its program to support rebels fighting against Syrian President Bashar al-Assad. 

General Tony Thomas denied that the four-year-old operation was brought to an end to placate Russia and earn its support for a ceasefire in southwestern Syria.

It was a "tough, tough decision" but "absolutely not a sop to the Russians," Thomas said at a forum in Aspen, Colorado. 

"It was, I think, based on an assessment of the nature of the program, what we're trying to accomplish, the viability of it going forward."