A top diplomat on Friday condemned the move by the Gulf state of Qatar to let Syria's opposition open an "embassy" in Doha, calling the decision "hasty and irrational," Iran's official IRNA news agency reported.
"Qatar's theatrical act in giving the Syrian embassy to a group which is unelected is both hasty and irrational," the deputy foreign minister in charge of Arab and African affairs, Hossein Amir Abdollahian, was quoted as saying.
"The people of Syria will not allow others to decide their country's fate," he said of Tehran's key regional ally.
"It is in Qatar's interests to stop acting hastily and intensifying bloodshed among the Syrian people."
On Wednesday, Syrian opposition chief Ahmed Moaz al-Khatib and Qatari State Minister for Foreign Affairs Khaled al-Attiya inaugurated a representative office dubbed the "Embassy of the Syrian National Coalition" in Doha.
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The original Syrian embassy itself in Qatar remains closed.
The opening came a day after opponents of President Bashar al-Assad, whom the Islamic republic staunchly backs, were given Damascus's long-vacant seat at the Arab League annual summit of the 22-member bloc in Doha.
Iran criticised the League move as a "dangerous precedent".
Sunni-ruled Gulf Arab states have long had strained relations with Shiite Tehran over what they allege is Iranian intervention in their domestic affairs.
In March last year, they announced the closure of their missions in Syria over the crackdown on dissent there that became a civil war which the United Nations estimates has killed more than 70,000 people.
Tehran, like Damascus, looks upon many Syrian opposition groups as "terrorists" backed by Western and Arab states, but supports dialogue to form a national reconciliation committee aimed at ending the conflict.