The commander of the rebel Free Syrian Army (FSA) said on Tuesday his group would shun a planned peace conference in Switzerland in January and would pursue its fight to topple President Bashar Assad regardless.
General Salim Idriss's stance highlights how hard it will be for international mediators to get Syria's warring and divided parties to the negotiating table in Geneva.
The "Geneva 2" conference will convene on Jan. 22, the United Nations said on Monday, with the stated goal of agreeing a transitional government to end a 2-1/2-year-old conflict that has killed well over 100,000 people and displaced millions.
"Conditions are not suitable for running the Geneva 2 talks at the given date and we, as a military and revolutionary force, will not participate in the conference," Idriss said.
"We will not stop combat at all during the Geneva conference or after it, and what concerns us is getting needed weapons for our fighters," he told Al Jazeera television.
The diplomacy has brought no let-up in the violence.
A suicide bomber killed 15 people and wounded more than 30 at a bus station in a suburb west of Damascus, state media said.
Heavy fighting has raged for days outside the capital Damascus and the northern city of Aleppo, with rebels reporting small gains after months of losing momentum to Assad's forces.
Assad, emboldened by a string of military successes, has said he will send delegates to the Geneva talks but will accept no preconditions and will put any agreement to a referendum - a vote which opposition figures say will be rigged against them.
The Western-backed FSA is an umbrella group encompassing many rebel units, but opposition sources and analysts say its influence has already been eroded by Islamist groups which are forging alliances among the most powerful rebel forces.
A senior Russian diplomat said Geneva 2, which was initially proposed by Russia and the United States in May, was delayed for so long because Washington wanted more time to try to organise an opposition delegation.
"The main problem ... remains the question of the format of participation of the Syrian opposition," Itar-Tass quoted Deputy Foreign Minister Mikhail Bogdanov as saying. "For precisely this reason, our American partners asked for additional time to complete the process of forming this delegation."
The Syrian National Coalition, a grouping also supported by the West, has announced conditional readiness to attend Geneva 2, despite objections from fighters and activists inside Syria.
Opposition figures in Istanbul said some coalition members were in talks in Geneva with the U.N. envoy for Syria, Lakhdar Brahimi, as well officials from the United States and Russia.
The coalition spokesman, Khaled Saleh, said it would meet again on Dec. 15 to decide whether to attend the talks.
He said the group wanted to know whether Assad and foreign powers would meet its demands for the creation of humanitarian aid corridors and the release of political prisoners.
"If they made sure the conditions for Geneva were there, we could have held Geneva tomorrow. Instead of focusing on the success of Geneva, they were focusing on the date," he said.
Anti-Assad demonstrations began in March 2011. Their violent suppression ignited a conflict that has split Syria among ethnic and religious factions backed by competing foreign powers. It has also inflamed regional Sunni-Shi'ite sectarian tensions.
The mostly Sunni Muslim rebels are wary of the Geneva conference, which they fear will not ensure Assad's departure.
In the opposition-held suburbs of Damascus, activists said rebels were advancing slowly near the international airport road against heavy resistance from Assad's forces backed by Shi'ite Lebanese Hezbollah fighters and the Abu Fadl al-Abbas brigades, a militia made up of foreign Shi'ite militants.
Foreign Sunni militants, many of them linked to al Qaeda, are also fighting alongside Assad's opponents.
Rebels east of Damascus are struggling to break a blockade that has cut off most food, supplies and weapons for six months.
Iran to attend if invited
The involvement of neighbouring powers in the conflict risks prolonging the violence and spreading it beyond Syria's borders.
Shi'ite Iran supports Assad, whose Alawite sect derives from Shi'ite Islam. Sunni Saudi Arabia is backing the rebels.
It is unclear if Iran, which struck a preliminary nuclear deal with world powers on Sunday, will attend the Syria talks.
Iranian Foreign Minister Mohammad Javad Zarif said on Tuesday his country would take part if invited, adding that it would in any event work for a peaceful solution.
"Participation of Iran in Geneva 2 is in our view an important contribution to the resolution of the problem. We have said all along that if Iran is invited, we will participate without any preconditions," Zarif told Iran's Press TV.
Western powers have questioned whether Iran should be invited because it has yet to endorse the outcome of a 2012 Geneva conference which forms the basis for the January talks.
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