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Russia's Foreign Minister Sergei Lavrov criticized the withdrawal of the invitation.
"The absence of Iran isn't going to help strengthen the unity of the world's Muslims," Lavrov said. But he still voiced hope for the talks.
"There is no catastrophe. We will push for a dialogue between the Syrian parties without any preconditions," he said.
Geopolitical obstacles to peace were compounded earlier in the week by comments from Assad, who told the AFP news agency on Sunday that he was likely to run in June's presidential race. He said in an interview there was a "significant chance" he would be a candidate.
"I see no reason I shouldn't stand," he said. "If there is public opinion in favor of my candidacy, I will not hesitate for a second to run for election."
His statement could further undermine a successful outcome for the Geneva conference. Opposition groups have said there is no chance they would accept any resolution at the Wednesday conference that did not involve a transitional government in which Assad had no role.
Other rebel factions, including the Islamic Front, a powerful coalition of Islamist groups,have refused to attend.
"We continue in our revolution, and we will not accept any political solution before the realization of certain terms," the Islamic Front said in a statement released Monday afternoon. The coalition said it was demanding the release of political prisoners, the resignation of the Assad regime, the exit of all sectarian militias from Syria and a guarantee that foreign powers would not intervene after Assad steps down.
Assad countered, "The Geneva conference must lead to clear results regarding the fight against terrorism. That would be the most important result of the conference. Any political result that did not include the fight against terrorism would have no value."
Assad exclusively refers to the rebels fighting to overthrow his regime as terrorists.
In line with that characterization, sources in Damascus told Al Jazeera on Monday that the Syrian government will demand support from the international community for its "fight against terrorism" in opening remarks at this week's talks.
Foreign Minister Walid al-Muallem, who heads the government's 15-member delegation, will also agree to open humanitarian corridors in besieged areas and agree to a ceasefire in Aleppo, the sources said.
Assad has ruled out that the Istanbul-based SNC could be given any ministerial positions in a new government, however, calling it "totally unrealistic."
"They ... come to the border for a 30-minute photo opportunity, and then they fled. How can they be ministers in the government?" he asked. "These propositions are totally unrealistic, but they do make a good joke.”
Al Jazeera and wire services
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