There is still no date for a meeting that aims to form a coalition of Syrian opposition groups before peace talks targeted for January 1, the Saudi foreign minister said Thursday.

Riyadh is trying to bring "as broad a cross-section of Syrian opposition groups as possible" to a meeting in the kingdom, Adel al-Jubeir told a press conference with his Austrian counterpart Sebastian Kurz.

Those talks would be a prelude to potential negotiations between the opposition and representatives of President Bashar al-Assad's regime.

In mid-November, diplomats from 17 countries met in the Austrian capital to seek a route toward a political transition in Syria, where nearly five years of war have killed more than 250,000 people.

"I can't give you a date" for the talks, Jubeir said.

"We're trying to do it sooner rather than later. But we need to first make a decision that it is feasible and productive to have these talks.

"We want to have all the minorities represented; we want to have all of the political groups represented," added the minister, whose country supports some Sunni forces on the battlefield and has bombed Islamic State group targets in Syria as part of a US-led coalition.

Groups on "terrorist lists", such as IS, would not be part of the talks, Jubeir said.

"We have reviewed several lists of candidates that were provided by several countries, and we have our own list," he said.

Once a decision is made to proceed, the details will be discussed with the United Nations and allies, Jubeir added.

The aim will be to unify Syria's opposition and clarify their objectives, and to strengthen their position with respect to potential talks with the Assad regime, he said.

Jubeir added that there are only two choices for Assad -- either he steps down peacefully "or he could be ousted militarily, but he has no future in Syria."

At the Vienna meeting, diplomats agreed on a target of six months to have a transitional government followed by elections within 18 months after that.

Opposition members have called the plan unrealistic, though some reacted with cautious optimism.

US Secretary of State John Kerry visited Abu Dhabi this week to encourage his Emirati and Saudi allies in their efforts to convince Syria's rebel factions to agree a ceasefire with Assad.