The Syrian peace conference will fail if Iran, Syria's main regional ally, does not participate, Iranian President Hassan Rouhani was quoted by news agency ISNA as saying.

"Any trans-regional meeting that is not attended by influential players will be unable to solve the Syria crisis," Rouhani told Russian President Vladimir Putin in a phone call.

Putin stressed the importance of Iran's participation at the conference.

"We believe any preconditions for Iran's participation are useless and unconstructive," Putin was quoted as saying, in a reference to Tehran's refusal to sign up to a June 2012 accord calling for a transitional government to replace the Assad regime.

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