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.

At a news conference in Jerusalem, U.S. Secretary of State John Kerry signaled a shift in tone when he talked of Iran playing a constructive if limited role at the conference even if it does not endorse the 2012 agreement.

Kerry, however, reiterated US 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.

Washington and Syrian opposition groups 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 near 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.

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 outlined a possible role for Iran at his press conference on Sunday.

"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 (UN) secretary-general, it has to be determined by Iranian intentions themselves," he told reporters in Jerusalem, where he is trying to broker peace between Israel and the Palestinians.

"But in terms of a formal invitation or participation -- that is for those who support the Geneva 1 implementation," said Kerry who later in the day flew to Amman for talks with King Abdullah II of Jordan.

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 June election of President Hassan Rohani 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.