Syria's Western-backed rebels are threatening to skip this week's peace conference after the United Nations invited Iran to attend.
A Syrian National Coalition spokesman said Iran has been "invading" Syria with its Revolutionary Guard and Hezbollah forces, so Iran's involvement in negotiations would be a deal-breaker.
The goal of the talks is to end Syria's bloody civil war.
In the lead-up, the Syrian regime has offered to implement a cease-fire and exchange detainees with rebel forces in Aleppo, one of the most contested cities.
If successful, this plan could be used as model for other parts of the war-torn country.
Foreign Minister Walid al-Moallem said the move is a confidence-building measure ahead of the Geneva II Peace conference next week.
The talks are expected to bring together members of the Syrian government and opposition forces for the first time. The goal is to find a political solution to the nearly 3-year-old civil war.
The United States has been urging the Syrian opposition to attend the conference. Stressing their attendance is vital to their credibility and also expressing the concern for the rise of extremism in Syria.
"The world needs no reminder that Syria has become the magnet for jihadists and extremists. It is the strongest magnet for terror of any place today. It is important that there be no games played with this process," U.S. Secretary of State John Kerry said.
Kerry also said the U.S. will continue to fight for the exchange and release of captive journalists and aid workers in order to try to improve the climate for negotiations.
Meanwhile, the United Nations is renewing its call on all nations to take in Syrian refugees.
"This is a responsibility of the international community. And that is why we have been strongly appealing for all borders everywhere to be open to Syrians, not only the borders of the neighboring countries," U.N. High Commissioner for Refugees Antonio Guterres said.
According to the Unitsed Nations, more than 2 million Syrians have fled the country since war broke out.