Terming the threats "outrageous and unacceptable," Foreign Minister Sergei Lavrov called for "the prompt intervention of those with influence over the opposition," RIA Novosti reported.
Some 19 armed Islamist groups battling the regime of Bashar Assad said in a joint statement Sunday that participating in the talks would be "treason," and that attendees "would have to answer for it before our courts."
Lavrov did not name any particular groups.
A number of opposition groups have indicated they will not attend the conference, set for Nov. 23 in Geneva. Assad has hinted his government may not attend either.
Lakhdar Brahimi, the U.N.-Arab League envoy to Syria, arrived in Damascus Monday to talk with regional leaders in advance of the conference, the Kuwait news service KUNA reported.
During his two-day visit, Brahimi is expected to meet with Assad and senior officials about possible resolutions to the Syrian conflict, which is 2 1/2 years old. He also is expected to meet with Syrian opposition leaders.
The Damascus stop was the last leg of Brahimi's tour of the Middle East, which has included stops in Egypt, Jordan, Kuwait, Iraq, Turkey, Qatar, Oman, Lebanon and Iran.
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