"We will not be able to hold the conference within the initially given time frame," Nikolai Patrushev said in an interview with the official Rossiyskaya Gazeta newspaper.
"Progress is slow," he added.
"A lot depends on the will and ability of the United States and a series of other countries to consolidate the (Syrian) opposition and to convince them to take part in this international forum."
Russia, one of the Syrian regime's staunchest allies, insists that "all actors" in the nearly three-year civil war must join the talks, dubbed "Geneva 2", which are to open on January 22.
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Also on Monday the main opposition Syrian National Coalition said it would not attend the talks if government warplanes keep up their bombing campaign on the northern city of Aleppo.
Yesterday, the Aleppo air blitz that began on December 15 had killed at least 422 people, mostly civilians, according to the Syrian Observatory for Human Rights, a Britain-based group relying on activists and other sources inside the war-torn country.
The Geneva talks are aimed at reaching an agreement on a political transition to end the war, which has claimed an estimated 126,000 lives since March 2011 and displaced millions.