"We are hostile to their presence and we have not invited them," Zarif said, according to the Tasnim news agency.
That goes against the position of the other two organisers of the talks - Russia and Turkey - which have said the new US administration of Donald Trump should be represented in Astana on Monday. The negotiations mark the first time since the conflict began in 2011 that the US has not been at the centre of peace negotiations.
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"At this stage, we must keep the tripartite set-up. Any enlargement could increase the risk of failure. Our policy is to not add other countries at this stage," Iran's foreign ministry spokesperson Bahram Ghasemi told AFP on Wednesday.
The talks come in the wake of President Bashar al-Assad's symbolic victory last month in retaking Aleppo, Syria's second city and a key rebel stronghold through much of the war. Talks were ongoing between Iran, Russia and Turkey on who would attend, Ghasemi said, and other countries could be included in later stages after successful "first steps".
"The meeting will not be at the ministerial level. It will probably be at the deputy minister level," he said. Ghasemi denied any major differences with Moscow over the involvement of Washington. Iran and Russia have been the key diplomatic and military backers of Syria in the war.
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"We have been working alongside each other for a long time, and have some strong convergences. There could at a certain stage, be some differences on certain subjects, but given the type of relations we have, we are sure to put ourselves in agreement through discussion," Ghasemi said.
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