UN Secretary-General Ban Ki-moon has invited Iran to participate in upcoming peace talks on Syria, a move that immediately angered the opposition, CNN reports.

The invitation to this week's opening-day session of the so-called Geneva 2 came as he held a surprise news conference at UN headquarters Sunday, Jan 19 night.

"As I have said repeatedly, I believe strongly that Iran needs to be part of the solution to the Syrian crisis," Ban said.

The Secretary-General added that he has spoken "at length" with Iranian Foreign Minister Javad Zarif in recent days.

"He has assured me 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 June 30, 2012 Geneva Communique, including the Action Plan.

"Foreign Minister Zarif and I agree that the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers. It was on that basis that Foreign Minister Zarif pledged that Iran would play a positive and constructive role in Montreux," Ban said.

Talks are due to start in the Swiss city Wednesday.

A spokesman for the Syrian National Coalition, the country's main opposition group, reacted angrily to the news but stopped short of saying that his organization would pull out of talks.

"This is a deal breaker for the National Coalition. If they insist to bring Iran, we will have to go back to the drawing board again," Louay Safi told CNN by phone from Turkey.

"We don't want to negotiate with Iran, we want to negotiate with the regime. Before we go (to the conference), Iran has to say they will pull out all the fighters (inside Syria) allied with it," Safi said. The United States similarly expressed concern about the Secretary-General's invitation.

"The United States views the UN Secretary General's invitation to Iran to attend the upcoming Geneva conference as conditioned on Iran's explicit and public support for the full implementation of the Geneva communique including the establishment of a transitional governing body by mutual consent with full executive authorities. This is something Iran has never done publicly and something we have long made clear is required.

"We also remain deeply concerned about Iran's contributions to the (President Bashar) Assad regime's brutal campaign against its own people, which has contributed to the growth of extremism and instability in the region. If Iran does not fully and publicly accept the Geneva communique, the invitation must be rescinded," said State Department spokeswoman Jen Psaki.

The goal of the talks -- dubbed Geneva 2 in recognition of a similar effort in the Swiss city in 2012 -- is to set up a transitional government that would end the violence that has wracked the country since March 2011, killing more than 100,000 people.

At least 155 people were killed Sunday, according to the Local Coordination Committees of Syria, a network of opposition activists. Nearly half of the deaths, 70, occurred in Aleppo, the group said.