President Obama vetoed 50 plans put to him by the CIA to engineer the downfall of the Assad regime in Syria, according to a former operative working on the project.

According to a memoir he is publishing on Tuesday, Douglas Laux was part of a team tasked to find ways to put into effect Mr Obama’s assertion in August 2011 that “the time had come for President Assad to step aside”.

The CIA, under then-leader David Petraeus, ended up running a scheme to arm rebels from the “non-jihadist” Free Syrian Army - but it never reached a scale that outweighed regime support from Iran and the Lebanese militia Hizbollah.

Mr Laux now says that was because more elaborate schemes drawn up and backed not only by former General Petraeus, Hillary Clinton when secretary of state, and defence secretary Leon Panetta were all rejected by Mr Obama.

"We had come up with 50 good options,” he said in an interview with the American television channel NBC. “My ops plan laid them out in black and white. But political leadership…hadn't given us the go-ahead to implement a single one."

Mr Obama has been accused of vacillating on Syria since the start of the uprising against President Bashar al-Assad, encouraging rebels by implying they had US support but failing to intervene decisively.