Iran could play a helpful role in finding a solution to the conflict in Syria even on the sidelines of peace talks in Geneva later this month, U.S. Secretary of State John Kerry said on Sunda.

It would be difficult to see how Iran, which did not support last year's international accord on Syria, could be a "ministerial partner" in the Geneva 2 talks slated to begin on January 22, Kerry said. But at a news conference during a visit to Israel he held out the possibility of Iran playing a constructive role on the sidelines even if it was not a formal participant.

"Now could they contribute from the sidelines? Are their ways for them, conceivably, to weigh in? Can their mission that is already in Geneva ... be there in order to help the process? It may be that there are ways that could happen," Kerry said.

Iran is the main backer, along with Russia, of Syrian President Bashar Assad in a conflict that has lasted more than two years, killed more than 100,000 people and uprooted millions more.

Tehran's participation in the peace talks would likely strengthen any agreement reached in Geneva, and Iran has said it was prepared to take part if invited.

But Washington has opposed Iran's attendance because it has not signed on to a core element of the so-called "Geneva 1" framework. This says a future Syrian government must be formed by "mutual consent" of the authorities and the opposition, a stance the United States says means Assad cannot stay in power.

Relations between Iran and Washington, frozen for decades, have improved sharply since the election in June of President Hassan Rouhani who promised to pursue a policy of "constructive engagement" with the West in order to win relief from international sanctions over Iran's nuclear program.