Leaders of two rebel brigades in the northern city of Aleppo, speaking to AFP after agreeing to join forces, also accused Russia and the regime of Bashar al-Assad of conspiring to buy time for the Syrian president to press his campaign against those seeking to overthrow him.
They were speaking as the United States and Russia hammered out a deal, based on proposals by Moscow, to eliminate Syria's chemical weapons by mid-2014 and leaving the door open to sanctions if Damascus fails to comply.
The battalion's commander, Abdulaziz Salameh, was even more scathing about the United States.
While accusing Russia and Syria of "devising a perfect plan to put a stop to an attack by the West," he said "Obama has shown himself not to be a man of honour, without anything to say."
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He added with bravado that "we don't need the attack. We don't need anyone. We count on the help of God, and he will guide us to final victory."
Abu Tawfiqa, a commander of the Liwa al-Fatah brigade, said "we don't want an attack, but if it happens the United States will find a very powerful ally on the ground."
The two brigades held a ceremony on Friday at a former military academy north of Aleppo to mark the agreement to unite their forces, which they claim number 13,000 across the country.
"The United States and Russia are playing with Syria; we mean absolutely nothing to them. The same is true of the United Nations, which accuses the regime of crimes against humanity and keeps talking and talking, as if that would solve anything. They have been talking for 30 months and done absolutely nothing."
A man standing nearby, interjected: "Obama is a liar. He won't keep his word. He said he would attack if Assad used chemical weapons against the civilian population. What more does he need to intervene and put an end to this slaughter."
Aleppo, once the commercial capital of the country, had been relatively untouched by the Syria conflict which has killed more than 110,000 people since it flared in March 2011.