However, United Nations Secretary-General Ban Ki Moon invited Iran to the conference, the second such meeting scheduled to start Wednesday in Geneva, without such a public statement. The U.S. State Department is trying to figure a way to keep the conference on schedule despite the latest dispute.
The Syrian National Coalition, Syria's main opposition group, said it was "suspending" its participation in the conference until Iran is disinvited or agrees to the road map for peace.
"We consider Iran a country that is invading Syria and sending in militia, whether it's Revolutionary Guard or Hezbollah," said Ahmad Ramadan, a senior member of the organization, which is based in Istanbul. "If the situation does not change, the coalition will not be" at the talks, he said.
However, other members of the Syrian National Council said they may attend the talks to counter the perception that Assad is willing to talk peace but the rebels are not.
"The first impression is that there is no benefit from Geneva II, everybody thinks that," said Hisham Marwa, a member of the Syrian National Council in Damascus. "But the SNC participating in the conference is to prove that the regime is not being serious about it."
The Syrian National Council recently voted in favor of participating – with 58 members out of 119 voting to attend and 44 boycotting the vote. The U.N.-hosted peace talks were formed after months of political maneuvering to get to a table members of Assad's government and rebel representatives.
Assad has been pounding civilians and rebel fighters in a nearly three-year civil war in which well over 100,000 people have died and 8 million people have been forced from their homes, many of whom must live in refugee camps outside the border. Assad has said the rebels are merely terrorist groups seeking to take over the state.
The uprising grew out of peaceful demands for democratic reforms and turned violent only after Assad directed his army to attack cities where opposition to his dictatorship was strong. Now, there are several U.S.-designated terror groups fighting alongside rebels, and sometimes against them.
Ban says Iran has accepted new elections in Syria, but it has not said whether it agrees that Assad should be banned from the elections.
Iran has been supporting Assad's regime with billions of dollars in credit and weapons since the conflict erupted in March 2011. Analysts said even if a diplomatic solution is reached it could take some time before there's real change on the ground.
"We've seen over and over again how forces on the ground inside Syria evolve rapidly and independently," said Mona Yacoubian, a senior adviser for the Stimson Center's Middle East program, in Washington.
"That's what's scary about Syria, particularly how we're seeing this proliferation of extremist groups, the influx of foreign fighters — all of these dynamics on the ground really have a life of their own," said Yacoubian.
Some Syrians say there will be no peace until Assad's regime is removed one way or another.
"Forming an interim government with some of regime figures means reforming the regime and not having new one," said Suzan Ahmad, a student and anti-regime activist in Damascus. "How do you think Syrians will accept the same regime that killed and displaced them to rule them again?"