US Secretary of State John Kerry said on Sunday Iran could play a role on the sidelines of Syrian peace talks, the first sign he might accept some participation from Tehran amid a thawing of relations between the two countries.
Washington, and Syrian opposition groups, have long objected to any involvement from Iran, the main regional ally of Syrian President Bashar al-Assad during the near three-year uprising against his rule.
Kerry, at a news conference during a visit to Israel, reiterated Washington's opposition to Iran being a formal participant in the negotiations, slated for Jan. 22, because it did not support last year's international agreement on Syria.
A core element of that so-called "Geneva 1" agreement effectively rules out Assad remaining in power.
But Kerry, in what appeared to be a change of tone toward Tehran, held out the possibility of Iran playing a constructive if limited role.
"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.
"But that has to be determined by the (UN) secretary-general, it has to be determined by Iranian intentions themselves. But in terms of a formal invitation or participation - that is for those who support the Geneva 1 implementation."
Two senior US officials said they believed it was the first time Kerry had publicly raised the possibility of Iran playing some kind of role at the talks without signing up to the Geneva 1 principles.
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.
World powers and Iran agreed in November on a landmark accord for Tehran to curb its disputed nuclear programme in return for a limited easing of sanctions.
"Iran could participate very easily if they would simply accept the Geneva 1 premise on which Geneva 2 is based," Kerry said, referring to a 2012 pact calling for the formation of a transitional government with the mutual consent of the Syrian government and opposition.
Along with Russia, Iran is Assad's main backer in a conflict that has lasted more than two years, killed more than 100,000 people and uprooted millions more.