An unexpected last-minute U.N. invitation for Iran to a peace conference on Syria threw the talks into doubt on Monday, with Washington demanding Secretary-General Ban Ki-moon withdraw his offer and the Syrian opposition threatening to pull out.

Iran is the main foreign backer of Syrian President Bashar al-Assad and its presence has been one of the most contentious issues looming over the first talks to be attended by both Assad's government and opponents.

The West and the Syrian opposition have long said Iran must be barred from the conference unless it first accepts an accord reached in Geneva in 2012 calling for a transitional government for Syria, which they see as a step towards unseating Assad.

Iran said it had done no such thing and had accepted Ban's invitation "without pre-conditions" -- the phrase it has long used to spurn the earlier accord.

Syria's main political opposition in exile -- the National Coalition, which agreed to attend the conference known as Geneva 2 only two days ago -- said it would announce it was withdrawing from the talks unless Ban revoked his invitation within hours.

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