Robert Ford, the former US ambassador to Damascus, criticised Barack Obama's policy on Syria, saying Washington should do more and take the initiative to arm the moderate opponents of the Syrian government.
In an interview with PBS NewsHour Ford said that as a result of US hesitancy, extremist threats to the United States had grown.
Ford, a veteran diplomat who is widely respected, served as ambassador in Damascus for more than three years until his retirement in March. He left the country in 2011 after the US received threats against his safety in Syria.
It is likely to fuel the debate about Obama's cautious attitude to the war at a time when the White House has launched a campaign to counter criticism of the president's foreign policy.
Syrians cast their votes on Tuesday in elections which resulted in the expected landslide victory for President Bashar Al-Assad. The elections were described as a farce in the midst of a devastating civil war in the country.
Ford said the election "is a signal, to us, to other countries in the region, to Europe, et cetera, that Assad is not leaving. He is staying deeply entrenched in the capital in Syria, even as other parts of the country remain outside his control."
Responding to Ford's comments, State Department spokeswoman Marie Harf said: "He's a private citizen. He's entitled to his views. What we're focused on today is the officials who are still here, who are working on Syria, who share the kind of frustration you've heard from the president, the secretary and others."
Obama called on Assad to step down from power in August 2011, but resisted calls for US intervention in Syria in August of last year. He also declined to guide missile strikes into Syria in response to frequent reports of the use chemical weapons by the regime. Instead, a US-Russian agreement was reached to transport Syria's chemical arms out of the country.
The United States has provided limited military training and supplies to the moderate opposition fighters. US officials said last month that Washington will expand its support for a select group of opponents, but gave no further details.
"It's not clear to me yet if they are prepared to ramp up [assistance] in a such a way that would be meaningful on the ground and that's what matters," Ford said in the interview.
"We need – and we have long needed – to help moderates in the Syrian opposition with both weapons and other non-lethal assistance.
"Had we done that a couple of years ago, had we ramped it up, frankly the Al-Qaeda groups that have been winning adherents would have been unable to compete with the moderates who frankly we have much in common with," Ford said.
Meanwhile, he said, Russia and especially Iran were massively increasing their assistance to Al-Assad.
"Our policy was not evolving and finally I got to a point where I could no longer defend it publicly," Ford said.