Western leaders on Friday called the defection of a general and childhood friend of Syrian President Bashar al-Assad a major setback for his regime and put pressure on Russia and China to also quit Assad's side, dpa reported.
News of the defection, which was confirmed by French Foreign Minister Laurent Fabius, came as representatives of around 100 countries and organizations met with opposition members at a Friends of Syria conference in Paris.
Fabius said the defection of "someone who belongs to the Republican Guard of Bashar al-Assad and was for a long time was one of his friends" showed that people within his close entourage were "beginning to understand his regime is not sustainable."
According to Fabius, Tlass - who was reported to have fled to Turkey - was now on his way to Paris.
US Secretary of State Hillary Rodham Clinton welcomed news of the defection, which is the most senior yet in the 16-month conflict and follows reports of dozens of soldiers deserting to Turkey in recent days.
"Those with the closest knowledge of Assad's actions and crimes are moving away and we think that's a very promising development," she said in Paris.
Tlass, a son of a former defence minister, is a Sunni Muslim from the restive city of al-Rastan in the central province of Homs, which is a key opposition stronghold.
According to military experts in Beirut, he was pushed aside after adopting a policy of negotiation with opposition rebels, a strategy that reportedly angered the regime.
It was not clear whether he would seek a role in the exiled opposition, which dominated previous Friends of Syria gatherings but this time shared the platform with militants from inside the country.
Notable absences from the conference, which repeated calls for Assad to leave power, were Russia and China. Both countries boycotted the talks.
Clinton took Syria's two allies on the United Nations Security Council to task over their continuing support for the regime.
"I don't think Russia and China believe they are paying any price at all, nothing at all, for standing up on behalf of the Assad regime," she accused.
Other countries should "not only urge but demand that they get off the sidelines and begin to support the legitimate aspirations of the Syrian people," she said.
To speed the transition, the group called for a Chapter VII Security Council sanctions resolution "that reaffirms (Syria envoy Kofi Annan's) six-point plan, endorses the Geneva communique including its plan for political transition and imposes measures under Article 41 of the United Nations Charter to enforce compliance."
Article 41 regulates non-military measures to enforce Security Council decisions, such as economic sanctions, blocking transport and communications, or the severance of diplomatic relations.
The conference also decided to "greatly increase assistance to the opposition" by giving them tools to communicate more securely with each other and the outside world, and urged humanitarian aid for people trapped by the fighting and support for Syria's future reconstruction.
But the Syrian opposition said it was disappointed with the outcome of the conference, saying it needed more than promises.
"The conference gave us moral and political support, but at this point we need more than promises from the international community," George Sabra, a spokesman for the Syrian National Council, told dpa from Paris.
Sherif Shahadeh, a pro-al-Assad lawmaker, called the conference "useless."
"Such conferences will do no good without Syria, Russia and China," he said in a televised comment from Damascus. "Only dialogue will lead to the end of violence."
In Geneva, the UN Human Rights Council adopted a resolution Friday envisaging a post-Assad Syria, by calling on all Syrians to deal with rights violations through reconciliation and legal processes.
Meanwhile, at least 55 people were killed Friday in Syria, including 14 pro-government soldiers, according to the opposition.
The soldiers were killed in clashes with rebels, said the London-based Syrian Observatory for Human Rights.
Eight air defence personnel defected from the Syrian army to join the opposition, the organization reported.
News from Syria cannot be independently verified, as authorities bar most media from restive areas.