The "Friends of Syria", an alliance of mainly Western and Gulf Arab countries who oppose Syrian President Bashar al-Assad, urged opposition groups to attend this month's peace talks, saying there was no other route to a political solution.
With 10 days to go until the first direct talks between the opposition and President Bashar al-Assad's government - set for 22 January in Switzerland and dubbed "Geneva 2" - Western backers have struggled to unify rebel groups.
The main political opposition body in exile, the Syrian National Coalition, has been plagued by bickering.
It postponed a decision on whether to attend until next week after nearly a quarter of its 121 members threatened to resign after the re-election of its Saudi-backed leader, Ahmad al-Jarba.
In a final statement, the 11 core Friends of Syria nations urged the SNC to attend the talks in Montreux, on the shores of Lake Geneva.
"There is no other political solution," French Foreign Minister Laurent Fabius said. "There will be no political solution for Syria unless 'Geneva 2' meets."
Mr Jarba, who attended the Paris meeting, did not say whether he believed the Coalition would go, but said he was reassured that the 11 nations had agreed that there could be no transition under Assad.
"We are all in agreement to say that Assad has no future in Syria," he said.
However, one of Geneva 2's main sponsors is Russia, which has shielded Assad from Western pressure on the UN Security Council and rejects the view that his departure is a necessary condition for a settlement.
US Secretary of State John Kerry is among those in Paris for the talks.
Mr Kerry will also take advantage of his stop in Paris to return to the issue of the Israeli-Palestinian peace process, with a meeting of Arab League nations.
Mr Kerry had been expected to return to Israel next week as he seeks to hammer out a framework to guide the negotiations in the coming months.
But his office said no decision had yet been made on when he might return for more talks with Israeli and Palestinian leaders.
The US State Department said the Paris meetings will keep him from travelling to Israel for tomorrow's funeral of former prime minister Ariel Sharon.
After Paris, Mr Kerry heads to Kuwait on 15 January for a Syria donor's conference set to be attended by officials from some 60 countries.
The conference aims to raise around €6bn to help Syrian civilians affected by the war.