The January 22 start of landmark Syria peace talks in Geneva will be difficult to keep as the war worsens and fallout spreads across the Middle East, analysts said.

"At long last and for the first time, the Syrian government and opposition will meet at the negotiating table instead of the battlefield," said UN leader Ban Ki-moon as he announced the date.

The conference would be the "best opportunity" to halt the bloodshed, according to US Secretary of State John Kerry, whose country helped broker the meeting with the UN and Russia.

But few observers see any chance of dousing the wildfires turning Syria into an inferno of conflict.

The conference guest list is not agreed, the international powers are divided and cannot control the carnage and bitterness between President Bashar al-Assad and the fractured Syrian opposition runs deep.

The UN says it can no longer estimate a death toll which it puts at well over 100,000. More than three million people will have fled to neighboring countries by the end of the year and UN envoy to Iraq, Nickolay Mladenov warned the UN Security Council on Monday of the growing spillover of extremists groups from Syria into Iraq where thousands have been killed this year.

Salman Shaikh, director of the Brookings Doha Centre think tank, said the fact that the United Nations has announced a date is already a positive sign.

But he put the chances of the Geneva meeting going ahead at only "50-50" and said it was "hostage to the situation on the ground."

"January 22 is still a long way off," said Richard Gowan, director of New York University's Center for International Cooperation.

"The Syrian army has been scoring new victories over the rebels, and could intensify its efforts to strengthen its military position before the talks."

UN-Arab League peace envoy Lakhdar Brahimi will meet with US and Russian officials on December 20 to try to sort invitations to Geneva.

Who will represent the rebels? Will the government delegation be empowered to take critical decisions? Should Iran, an Assad backer, and Saudi Arabia, supporter of the opposition, be included? "The answers to these questions will be critical," commented one UN diplomat following negotiations.

The opposition has to go, according to Shaikh. "It is weak and it needs the legitimacy that has been conferred on it by the international community. And they don't want to be the ones who are blamed for the failure of the talks by not showing up."

The rebels are "running a huge risk" by going, he added however. "If there are no real achievements and the situation on the ground is the same if not worse ... then of course this group of individuals from the opposition will be facing severe criticism."

Iran's presence would be controversial as it has not yet supported a declaration by the major powers from June, 2012 which called for a transitional government in Syria.

Ban and western leaders have stressed that the new conference must concentrate on how to carry out that declaration.

Gowan at NYU said the UN, United States and Russia want the conference to happen now a date has been set. "But there is still a high chance of the talks derailing once they begin. Or they could result in a very limp compromise if the negotiators are not serious."

"Is it going to be easy to get to January 22 without any hiccups? Probably not," said the UN spokesman Martin Nesirky acknowledging the obstacles. The conference will be the start of "a process," he added, downplaying expectations for immediate results.

Ban said both sides in the conflict "can and must begin now to take steps to help the Geneva conference succeed, including toward the cessation of violence, humanitarian access, release of detainees and return of Syrian refugees and internally displaced people to their homes."

The backers of both sides say the next few weeks will be a major test of the international community's ability to ability to put out the flames in Syria.