Special envoy Staffan de Mistura said more than 40 groups had been invited to attend one-on-one meetings over the next five to six weeks.
Iran and Turkey have also been invited, but jihadist militant groups have not.
Mr de Mistura described it as a stock-taking process, saying there would be no big roundtable discussions.
The last round of peace talks in Geneva collapsed in early 2014 with the government refusing to discuss opposition demands.
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