Syrian rebel fighters in an undated photo/via Flickr Creative Commons

Iran had initially been one of ten nations invited to take part in the peace talks, which are scheduled to start on Wednesday, but that invitation was later rescinded after the United States and other Western countries expressed anger at the decision.

The National Coalition, a main opposition group to the Syrian government, also threatened to pull out of the talks if Iran had been allowed to attend.  The Syrian government is reportedly receiving military and economic assistance from Iran.

A State Department spokesperson said that the U.S. was "deeply concerned" about Iran's support for the Assad government, and that "if Iran does not fully and publicly accept the Geneva communique, the invitation must be rescinded."

Iran had also reportedly rejected a June 2012 agreement for a political transition in Syria. A spokesperson for UN Secretary General Ban Ki-moon said that the global leader "continues to urge Iran to join the global consensus behind the Geneva communique."

"Given that it has chosen to remain outside that basic understanding," UN spokesperson Martin Nesirky said, "[Ban Ki-moon] has decided that the one-day Montreaux gathering will proceed without Iran's participation."

Nesirky also conveyed the Secretary General's disappointment that Iran had rejected the 2012 deal despite initial assurances by the Iranian government that they would support ending the Syrian civil war.

In response, Iran's UN Ambassador Mohammad Khazaee said that "if the participation of Iran is conditioned to accept Geneva I communique, Iran will not participate in Geneva II conference."

Read more about Iran and the Syrian crisis at BBC News, Reuters, and CNN.

Reach Executive Producer Colin Hale here. Follow him on Twitter.