Secretary of State John Kerry will lead the US delegation to key talks next week aimed at bringing together the Syrian regime and the opposition for the first time since the war erupted, a US official said Wednesday.

Intense preparations are under way ahead of the UN-led January 22 talks which open in the Swiss city of Montreux, with the Syrian opposition still to decide whether to attend or not.

"We're operating under the assumption that they will go. I don't want to venture to guess what would happen if they don't," said deputy State Department spokeswoman Marie Harf.

US ambassador to Syria, Robert Ford, was again meeting with the exiled opposition in Istanbul to try to persuade them to join the negotiations with a vote due on Friday, she said.

"The value would be greatly diminished, let's all be honest here, that if they don't go it would be, because it's in their interest to go," Harf said.

"It's not just that it's in our interest; it's that it's in their interest and the Syrian people's interest that they go."

But elements of the fractured opposition have balked at sitting down at the negotiating table with representatives of the Syrian regime of President Bashar al-Assad after three years of brutal conflict which has left more than 130,000 dead.

Kerry spoke Wednesday with Russian Foreign Minister Sergei Lavrov by telephone after face-to-face talks in Paris on Monday.

Lavrov and Kerry discussed "the preparations for the international peace conference on Syria and the measures aimed at easing the humanitarian crisis in the country," the Russian minister's office said in a statement.

Harf said a lot of behind-the-scenes meetings were already taking place to "create a climate" to give the peace conference -- which aims to put in place a process for a transitional government to replace Assad -- "more of a chance of success."

The talks will build on a June 2012 deal struck in Geneva setting out a path towards the transitional government.

The talks will also focus on getting aid groups access to areas badly hit or under seige in the conflict, as well as seeking a ceasefire.

Harf also said that Kerry would travel from Montreux to the Swiss city of Davos to address the annual meeting of the World Economic Forum about his drive for a Middle East peace deal.