Syria's main opposition bloc, the Syrian National Council, and the Free Syrian Army (FSA) rebels agreed on Thursday to co-ordinate operations against President Bashar al-Assad.
The Syrian National Council (SNC) also said the FSA had agreed to cut back attacks on government forces, after their first meeting in southern Turkey.
Analysts say the rapidly growing FSA, formed by Turkish-backed Syrian army deserters, had posed a dilemma to the bloc due to the SNC focus on non-violent opposition.
The announcement came on the same day that the top UN human rights official declared Syria to be "on the cusp of civil war" and estimated more than 4,000 dead in Syrian president Bashar al-Assad's bloody seven-month crackdown.
"We are placing the figure at 4,000, but really the reliable information coming to us is that it is much more than that," United Nations High Commissioner for Human Rights Navi Pillay told reporters.
"I have said that as soon as there were more and more defectors threatening to take up arms - I said this in August before the Security Council - there was going to be a civil war. At the moment that's how I am characterizing this," she said. Pillay's office later amended her position to say Syria was "on the cusp of civil war."
"I intend to add my voice to the finding of the commission of inquiry with regard to evidence pointing to the commission of crimes against humanity [by Assad's regime]," said Pillay, a former U.N. war crimes judge.
"In my own view, based on our own monitoring of the situation, there is need for prosecution of perpetrators at the highest level for crimes against humanity," she said on Thursday.
The regime of Bashar al-Assad has found itself under increasing pressure to end the crackdown and abdicate power by Western nations and his own Arab allies. He has also faced a growing FSA insurgency led by dissident Syrian generals from Turkey. The FSA is said to have at least 20,000 armed fighters with its ranks swelling every day.
Regional analysts say the collapse of Assad's regime could dramatically alter the geopolitical landscape of the Middle East.
Analysts say Hizbullah, which relies on Syria for logistical support may seek a coup in Lebanon should Assad fall as a means of keeping a growing domestic opposition from further aligning themselves with the West in order to isolate the Shiite terror militia.
Iran, they add, would also find itself seeking to influence Syria's future, quite possibly by supporting a terror insurgency of its own against Damascus, should Assad's possible successors seek rapprochement with the West.
Russia and China, which both have multi-billion dollar oil concessions in Syria, teamed up in October to veto a Western-backed Security Council resolution condemning Assad's government for violence.
The two powers, joined by Cuba, are also trying to tone down an EU resolution being presented on Friday that would strongly condemn Syria and call for the UN report on crimes against humanity to be sent to the Security Council, diplomats said.