It is the first time Iran has joined global discussions aimed at a peace settlement in Syria, having not been invited to similar meetings in 2012 and 2014 in Geneva.
"We have reviewed the invitation, and it was decided that the Foreign Minister would attend," Foreign Ministry Spokeswoman Marzieh Afkham said, without disclosing when Zarif would arrive.
Afkham did not say who issued the invitation but it followed comments in last two days in which a State Department spokesman said Iran would probably be asked to attend.
US Secretary of State John Kerry will leave Washington on Wednesday for the talks with 'around a dozen countries' on the crisis in Syria, officials said.
The talks are an expansion of discussions held last week in Vienna between the United States, Russia, Turkey and Saudi Arabia that did not reach any substantive conclusion.
The core talks are to take place in Vienna on Friday, but diplomats expect some preparatory or bilateral meetings in the Austrian capital from late Thursday.
After being left out of talks known as Geneva 1, Iran was initially invited to Geneva 2 talks, but UN chief Ban Ki-moon withdrew the invitation after the US and other powers voiced opposition to Tehran's inclusion.


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