Leaders of the National Coalition for Syrian Revolutionary and Opposition Forces voted Saturday to attend the talks in Switzerland despite low expectations they will lead to an end of the bloody rebellion against President Bashar Assad.
Kerry said in a written statement that the conference was "the best opportunity to achieve a political transition."
The United States and other international powers have pushed the Geneva conference as the best and possibly only way to end the fighting that has dragged on for three years.
The New York Times said Sunday that the opposition remained less confident. Opposition leaders meeting in Turkey voted 58-14 to attend the talks. Another 44 of the coalition's 119 active members said this weekend they would not vote and would skip the peace talks.
Some of the parties boycotting the talks said the idea of negotiating with Assad did not have support among the rebels on the front lines, the Times said. "This is a very big mistake," Nasr al-Hariri, a spokesman for the boycott group, said. "This decision will put us in conflict with the opposition on the ground."