Saudi Minister of Foreign Affairs Prince Saud al-Faisal bin Abdulaziz al-Saud, right, listens as Secretary of State John Kerry makes a statement to the press at King Khalid International Airport in Riyadh Jan. 5, 2014. Brendan Smialowski/Reuters

The United States on Sunday appeared for the first time to hold out the possibility that Iran might play a role on the sidelines of a Syria peace conference, even if Tehran is not formally invited.

Syrian opposition groups and Washington have long had reservations about the participation of Iran, which they accuse of supporting Syrian President Bashar al-Assad with manpower and arms during the nearly three-year uprising against his rule.

Despite the improvement in U.S.-Iranian relations this year with a landmark nuclear deal struck in November, ties are strained by many issues, including the Syrian civil war in which at least 100,000 people have been killed and millions uprooted.

At a news conference in Jerusalem, Kerry reiterated U.S. opposition to Iran being a formal member of the so-called "Geneva 2" talks scheduled for Jan. 22 in Switzerland, because it does not support a 2012 international agreement on Syria.

That so-called "Geneva 1" accord called for the Syrian government and opposition to form a transitional government "by mutual consent," a phrase Washington says rules out any role for Assad. Russia, a sponsor of the plan, disputes that view.

Kerry, in a shift in tone, held out the possibility of Iran playing a constructive, if limited, role at the conference even if it does not endorse the 2012 agreement.

"Now, could they contribute from the sidelines? Are there 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," he said.

"But that has to be determined by the (U.N.) secretary-general. It has to be determined by Iranian intentions themselves," he told reporters in Jerusalem, where he trying to nudge Israel and the Palestinians toward a peace agreement.

"But in terms of a formal invitation or participation — that is for those who support the Geneva 1 implementation."

Senior U.S. 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.

In the clearest sign of warming ties, six major powers and Iran reached an agreement on Nov. 24 designed to curb Iran's nuclear program in return for limited Western sanctions relief.

However, U.S. accusations that Iran supports international "terrorism" and is fueling the violence in Syria with armaments continue to divide Washington and Tehran.

"Iran could participate very easily if they would simply accept the Geneva 1 premise on which Geneva 2 is based," Kerry said, referring to the 2012 pact calling for the formation of a transitional government with the mutual consent of the Syrian government and opposition.

"We are happy to have Iran be helpful. Everybody is happy to have Iran be helpful," he added.

Reuters