George Mitchell, who stepped down from the role a year ago, said there were a number of options short of direct military action that should be assessed.
“A government which must kill now more than 10,000 of its own people to stay in power cannot be said to have the consent of the people,” said Mr Mitchell in an interview in Doha.
“In the end, the public will be vindicated and there will be a change and hopefully a mechanism for expressing their democratic will.”
About 10,000 people have been killed by the regime of president Bashar Al Assad since an uprising began 14 months ago, according to United Nations estimates.
Ban Ki-moon, the UN secretary general, has voiced fears that the conflict may be spreading to Lebanon, where at least two people were killed in the capital Beirut on Monday amid fighting between gunmen that followed the murder of an anti-Syrian cleric.
“I don’t favour direct military action. I think it would be counterproductive, but there are a range of things short of that which we should be doing. I’m confident the administration is exploring some of these,” said Mr Mitchell.
“Clearly there is a mechanism in the US for what we call covert operations to be engaged actively in a non-public way in pursing what is perceived to be a public good and there are many things we could be doing in that context.”
He added that he had no personal knowledge of any such existing US involvement in the country.
The former US Senate majority leader said the creation of humanitarian zones along the border between Syria and Turkey would also help those fleeing the violence.
A bomb exploded in Damascus killing five people yesterday, state media and activists reported.
Syrian police also killed two protesters yesterday when they opened fire on a crowd who came out to welcome UN observers in the eastern province of Deir Al Zor.
“As soon as the UN convoy entered Al Busaira, a jubilant crowd of hundreds came out to welcome them. It was not minutes before they came under fire,” said Abu Laila of the Free Syrian Army, the opposition force.
Conflicting reports about high-level assassinations of people connected to the regime have also emerged from Syria this week.
“There is definitely an increase in assassinations targeting people associated with the regime, be they officials or pro-regime businessmen,” said Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, on Monday.
“For most of human history the authority of government was supposedly divinely granted or acquired through sheer force,” said Mr Mitchell.
He served as US special envoy for Middle East peace from January 2009 to May 2011 and also helped broker the 1998 Good Friday agreement, ending decades of conflict in Northern Ireland.
Mr Mitchell added: “But the greatness of democracy has established that the only legitimate authority of government is the consent of the governed. It’s very hard for anyone to make the argument that the government of Syria is based on the consent of the governed.”
Source: The National