Iranian President Hassan Rouhani said Thursday a Syrian peace conference planned for later this month will fail if Tehran, Damascus's main regional ally, does not participate, media reported.
The remarks came ahead of a meeting Monday at which Russia and the United States are to discuss Tehran's possible involvement in the talks set to start in Montreux, Switzerland on January 22.
"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 by telephone, the ISNA news agency reported.
"Thus, the Geneva 2 conference has already failed without it even being started," he added, calling the Russian-US initiated forum "a negotiation show."
Invitations to Geneva 2, already delayed by months, have been sent to 30 possible participants.
But Iran, a staunch supporter of President Bashar al-Assad's regime, is not on the list, according to the United Nations.
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On Monday, the Islamic republic brushed aside a US suggestion that it play a "sideline" role at the negotiations, with foreign ministry spokeswoman Marzieh Afkham calling the proposal insulting.
Russia has been one of the principal backers of Iran's inclusion in the talks, which aim to engage representatives of the Syrian regime and members of the opposition in direct talks for the first time.
According to ISNA, Putin reiterated Moscow's support and expressed hope "to convince our allies… that Iran, as a key actor, must be present at the talks."
"We believe any preconditions for Iran's participation are useless and unconstructive," it quoted the Russian leader as saying. That was a reference to Tehran's refusal to sign up to a June 2012 accord calling for a transitional government to replace the Assad regime.
Iran says it supports a political solution to the crisis that has pitted Assad against mostly Sunni rebels.
Tehran is accused of providing military and financial support to Damascus, despite repeatedly maintaining that it has no official military presence in Syria and that its backing takes the form of humanitarian aid.