The first direct talks between the Syrian regime and the opposition, which start this week in Geneva, will last seven to 10 days and be followed by another round, a senior Russian official said on Tuesday.
"The first round of negotiations will last for seven to 10 days. There will then be a short break, and then the talks will resume," the Interfax news agency quoted a source in the Russian delegation to the talks as saying.
The so-called Geneva II conference begins on Wednesday in the Swiss lakeside city of Montreux, where representatives from nearly 40 regional and world powers will be seeking a way out of the nearly three-year Syria crisis.
US President Barack Obama called Russia's Vladimir Putin as the clock ticked down to the start of talks, the Kremlin said late Tuesday.
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The two leaders "discussed the issues of the conference," it said in a short statement. The discussion was "the initiative of the American side," it said.
The Montreux meetings will be followed on Friday by direct talks in Geneva between representatives of Syrian President Bashar al-Assad and the opposition, which will be overseen by UN-Arab League envoy Lakhdar Brahimi.
The Russian source said Friday's meeting will also be observed by US and Russian officials, who will be assisting the two sides.
"If it becomes necessary, the seniority level of the Russian and US representatives in Geneva observing the negotiating process may be raised," the RIA Novosti news agency quoted the Russian source as saying.
Russia is represented at the talks by Deputy Foreign Ministers Gennady Gatilov and Mikhail Bogdanov.