At the start of a crucial couple of weeks for attempts to end a civil war that has claimed 130,000 lives, Kerry joined ministers from 10 other countries in seeking to persuade the opposition National Coalition to attend a first round of talks in Montreux, Switzerland on January 22.
The Swiss talks have been organised in an attempt to revive a long-stalled framework for peace involving a cessation of hostilities and the creation of a national transitional government that could involve figures from the current regime and the opposition.
Coalition leader Ahmad Jarba has called for Assad to stop using heavy weapons, lift sieges on a number of opposition- held areas and allow the opening of humanitarian corridors as a show of good faith ahead of any talks.
There has been no sign of progress on those issues but US officials have expressed confidence that, with little prospect of securing a military victory after nearly three years of fighting, the opposition will come to Montreux.
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German Foreign Minister Frank-Walter Steinmeier said it was vital the opposition came to the negotiating table.
"I know that it is not an easy decision for the opposition in Syria," he said. "We want to work to convince them today in Paris and remove the last obstacles that may arise.
"We must get down to work in earnest. I fear that we will not be successful if we do not manage to include the opposition in these talks."
Skirmishes have left an estimated 500 people dead in a matter of days and ISIL is threatening to abandon frontline positions in the area around Syria's second city, Aleppo.
The fighting has exacerbated concern in western capitals over the strength of radical Islamist groups within the broad alliance of forces fighting Assad.