There are conflicting reports as to whether the powerful rebel group Ahrar al-Sham signed the statement. It had earlier complained that figures it regarded as too close to the government were being given too prominent a role, and pulled out.
But Reuters news agency said the group had signed the copy of the statement at the end of the two-day talks.
World powers want negotiations between a unified opposition delegation and President Bashar al-Assad`s government on a political solution to the four-and-a-half-year conflict to start by 1 January.
Opposition groups had previously demanded Assad leave before any negotiations took place. But the statement says he would be allowed to stay until a transitional government was formed.
This signifies "a big change" on behalf of the opposition, Ibrahim Hamidi, the Syria editor of the Saudi owned al-Hayat newspaper, told the BBC.
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