Ahmad Jarba, the leader of Syria's opposition National Coalition, says the US-led "Friends of Syria" grouping has agreed that President Bashar al-Assad and his family will have no role in the country's future.
Jarba did not announce whether or not the opposition would take part in peace talks with representatives of Assad's regime due to start next week in Switzerland.
The Coalition, which is under intense pressure to confirm its participation, has said it will decide on the issue on January 17.
"We are at a crossroads today in the framework of international decisions regarding the Syrian revolution," Jarba said.
"We have passed a milestone on the way to the end of the regime."
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French Foreign Minister Laurent Fabius implicitly acknowledged that it was still uncertain that the opposition would be represented at the so-called Geneva II peace talks scheduled to start on January 22 in Montreux.
"It is important that the Geneva II meeting takes place and succeeds," Fabius said.
"The only solution to the Syrian drama is a political solution."
In a statement issued at the end of Sunday's talks, the 11-member Friends of Syria urged the opposition to respond positively to UN Secretary General Ban Ki-moon's invitation to send a delegation to the Montreux talks.
