“This is a way to try to balance the Russian game and make it clear that the Europeans want to play ball around Geneva, but have this option open,” said Camille Grand, director of the Foundation for Strategic Research in Paris. “The message to both Assad and Moscow is that, ‘You’re not winning on all fronts, and we have a plan B that would make your military successes more difficult, that we can also play the protracted war scenario.’ ”

The jockeying, however, may scuttle the long-shot peace talks even before they get off the ground. Foreign Minister Sergey V. Lavrov of Russia said that the European Union decision was “illegitimate in principle.”

“A number of actions that are being taken — and they are not being taken without the involvement and support of our Western partners, including the U.S. and France — intentionally or unintentionally serve to disrupt the conference,” Mr. Lavrov said, according to the Interfax news service.

The deputy foreign minister, Sergei Ryabkov, in a statement called the European Union decision “a reflection of ‘double standards.’ ” Later, he said, “You cannot declare the wish to stop the bloodshed, on one hand, and continue to pump armaments into Syria,” according to Interfax.

A senior European official, told of the comment, said that Russia, which has itself been pumping arms into Syria, might “take its own words to heart.”

Mr. Ryabkov, who has been seen as collegial to Western diplomats in the nuclear talks with Iran, insisted that Russia, by contrast, was selling arms to “legitimate authorities,” not supplying rebels. He defended a plan to provide Syria with advanced S-300 air defense missiles, saying that they would be a “stabilizing factor” that could deter a Western-led intervention.

Russia, on the request of the United States, Europeans and Israel, has not yet delivered those missiles to Syria. Defense Minister Moshe Yaalon of Israel said Tuesday that if the S-300s “arrive in Syria, we will know what to do,” suggesting that would again threaten an escalation with another Israeli strike.