Kerry and other envoys from the so-called “Friends of Syria” group are to meet the Syrian opposition in Britain on Tuesday to review progress toward convening the conference.
The National Coalition, Syria’s main opposition bloc, said it would hold internal discussions next week to decide whether it would attend the talks.
The Syrian National Council, a key member of the coalition, has already said it opposes the talks and threatened to quit if the umbrella group takes part.
The international community has been urging the rebels and the regime for months to participate in talks on a negotiated solution to the conflict.
However, al-Assad’s government says his departure from office will not be on the table, while the opposition insists he cannot remain in power.
The renewed push for talks comes after a September deal under which Syria agreed to turn over its chemical arsenal for destruction.
Inspectors from the Organisation for the Prohibition of Chemical Weapons, which won this year’s Nobel Peace Prize, said on Friday they have visited 14 out of more than 20 sites in Syria.