General Selim Idriss, the head of the rebel Free Syrian Army, said meanwhile that he was ready to go to the talks in Geneva if conditions including the fall of Assad were met.
The Coalition said it "affirms its absolute rejection of Assad or any of the criminals responsible for killing the Syrian people playing any role in a transitional body... or in Syria's political future."
The statement came a day after UN Secretary General Ban Ki-moon announced that peace talks, dubbed Geneva II, would be held on January 22, after several delays.
The Coalition said it considers the fact a date has been set as "very positive".
But it also said that in preparing for Geneva II the international community must "ensure humanitarian supplies reach all areas of Syria, while all prisoners must be set free".
It also called for "an immediate end" to massacres in Syria.
The new talks must "implement all the provisions of Geneva I", which in June 2012 envisaged the creation a transitional government for Syria, without stipulating that Assad should step down.
The Geneva II conference is a US-Russian initiative, and if it goes ahead it will bring rebel and regime representatives to the negotiating table.
Speaking to Al-Jazeera television, General Idriss of the Free Syrian Army said: "If we receive proposals that lead to the realisation of the demands of revolutionaries on the ground, we will take part."
Idriss, who heads the FSA's Supreme Military Council, also said no one had consulted with his group, "neither on the date nor the delegations set to attend the conference."
The foreign-based rebel command "will not take part if there are no guarantees regarding our... demands" that neither Assad nor "the gang of killers that surrounds him" have a role in Syria's future.
"We will not stop fighting, either before or after the conference," Idriss added, whose command has lost control on the ground. He also said he believed "the conditions for (the conference) have not yet been met".
In Damascus, dissident Hassan Abdel Azim told reporters the Syria-based opposition tolerated by the Assad regime was willing to go to the talks in a single delegation alongside Coalition members.
"There are those in the Coalition who, like us, want a political solution. We propose that we set up with those people a single delegation," said Abdel Azim, who heads the National Coordination Committee for Democratic Change.
"We are ready to coordinate with Coalition dissidents who want a political solution, and to establish alongside them a common vision and mechanisms for negotiation," he added.
Several Islamist battalions fighting Assad's forces had in September warned against any negotiations with the regime, and said anyone who did go to such talks would be branded a traitor.