ANKARA/BRUSSELS - Iran on Monday appeared to rule out participation in Syrian peace talks later this month, dismissing a US suggestion that it could be involved "from the sidelines" as not respecting its dignity.

US Secretary of State John Kerry suggested on Sunday there might be ways Iran could "contribute from the sidelines" in a so-called Geneva 2 peace conference in Montreux, Switzerland, on Jan 22, and on Monday US officials said Tehran might still be able to play a helpful role.

United Nations Secretary-General Ban Ki-moon was sending out invitations on Monday to potential participants the talks, but while he wants Iran to attend there was no agreement yet on whether to invite it.

The key players in the talks are President Bashar al-Assad's government and opposition rebels who have been fighting for nearly three years to oust him.

Syrian opposition groups and Washington, which accuse Tehran of supporting Assad with manpower and arms during the uprising against him, have long had reservations about the participation of Iran, although the United Nations special envoy on Syria, Lakhdar Brahimi, has backed Tehran's involvement.

While there has been a warming in US-Iranian ties this year including a November 24 deal to curb the Iranian nuclear programme, there are no visible signs that this has led to greater improvement in other areas such as Syria, where they are on opposite sides of the civil war.

Kerry reiterated US opposition to Iran being a formal member of the peace talks because it does not support a 2012 international agreement on Syria.

That "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.

On Monday, US officials said Iran could improve its chances of playing a role on the sidelines of the Syria peace talks by working with Damascus to stop the bombardment of civilians and improve humanitarian access.

"There are ... steps that Iran could take to show the international community that they are serious about playing a positive role," one of the officials said in Brussels.

"Those include calling for an end to the bombardment by the Syrian regime of their own people. It includes calling for and encouraging humanitarian access."

Another official made clear that the comment on bombardment referred to Syria's biggest city, Aleppo, where dozens have been killed in raids in which the Syrian air force has used improvised barrel bombs.