UN Secretary-General Ban Ki-moon said on Sunday he had invited Iran to participate in Syria peace talks on January 22 in Montreux, Switzerland and that Tehran had pledged to play a "positive and constructive role" if it was asked to participate.

Reacting to the news, Syria's main, Western-backed opposition said it will not attend if Iran comes to the gathering. Ahmad Ramadan, a senior member of the Syrian National Coalition, said the opposition is "suspending" its participation because Iran is "invading" Syria. "If the situation does not change, the Coalition will not be" at the talks this week, Ramadan said.

In a press release issued by the United Nations, Ban said he had spoken with Iranian Foreign Minister Javad Zarif at length over recent days and that the later had assured him that "like all the other countries invited to the opening day discussions in Montreux, Iran understands that the basis of the talks is the full implementation of the 30 June 2012 Geneva Communique, including the Action Plan."

"Foreign Minister Zarif and I agree," the press release continued, "that the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers."

Ban went on to say that Zarif pledged that Iran would play "a positive and constructive role" in Montreux.

In response the United States said on Sunday that the invitation should be withdrawn unless Tehran publicly states its support for a 2012 agreement that establishes a transition government in Syria.

"If Iran does not fully and publicly accept the Geneva communique, the invitation must be rescinded," State Department spokeswoman Jen Psaki said in a statement.

The United States has long said that Iran should not be allowed to attend the talks in Switzerland on Jan. 22 until it states its support for a June 2012 agreement for a political transition in Syria.

Earlier this month U.S. Secretary of State John Kerry reiterated U.S. 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.

Earlier this month Iran appeared to rule out participation in the Syrian peace talks, dismissing a U.S. suggestion that it could be involved "from the sidelines" as not respecting its dignity.

In remarks quoted by state television, the Iranian foreign ministry's spokeswoman said Tehran supported a political solution to end the Syrian civil war, in which at least 100,000 people have been killed and millions uprooted.

"But in order to take part in the Geneva 2 conference, the Islamic Republic of Iran will not accept any proposal which does not respect its dignity," the spokeswoman, Marzieh Afkham, was quoted as saying.

The key players in the talks are President Bashar 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 U.S.-Iranian ties including a November 24 deal to curb the Iranian nuclear program, 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.