The head of the al-Nusra Front says the al-Qaeda affiliate will not accept the outcome of next month's talks between Syria's government and opposition.
In his first television interview, Abu Mohammed al-Julani told al-Jazeera he would "not recognise any results that come out of the Geneva II conference".
Julani also claimed that the rebels would "achieve victory soon" against President Bashar al-Assad's forces.
Al-Nusra, which is designated a terrorist group by the US, is believed to have been created in mid-2011 with the help of the Islamic State of Iraq (ISI), a militant umbrella group that includes al-Qaeda.
It declared its existence in January 2012 and has since emerged as one of the most effective rebel forces, operating in most of Syria's provinces.
In April 2013, ISI's leader announced the merger of his group and al-Nusra, creating the Islamic State in Iraq and the Levant (ISIS). But Julani rejected the move and asserted his allegiance to al-Qaeda's overall leader, Ayman al-Zawahiri, who recognised al-Nusra as its Syrian offshoot.
Since then, al-Nusra and ISIS have operated as separate entities, with the former having an estimated 5,000 to 7,000 fighters and maintaining better relations with other Islamist rebel groups.
Not much is known about Julani, other than that was imprisoned by the Syrian authorities after taking part in the insurgency in neighbouring Iraq but released in early 2011 as part of an amnesty.
He asked that his face be hidden during his conversation with al-Jazeera's Tayseer Allouni, who said the security checks he went through were stricter than those carried out before he interviewed Osama Bin Laden in 2001.
Despite the rebels suffering a series of defeats in recent months, Julani declared: "The battle is almost over, we have covered about 70% of it, and what's left is small. We will achieve victory soon. We pray to God to culminate these efforts with victory. It's only a matter of days."
He also reiterated that al-Nusra did not recognise the Western-backed National Coalition, which has said it is prepared to attend talks to find a political solution to the 33-month conflict.
"We will not recognise any results that come out of the Geneva II conference, nor will the children or women of Syria do. Those taking part in the conference do not represent the people who sacrificed and shed blood. Besides, who has authorised them to represent the people?"
"Those are confined to newsrooms. In reality, they have no presence on the ground. We cannot allow the Geneva II game to fool the nation, to push us back 50 or 100 years."
Julani said al-Nusra would "not seek to rule society on its own" once Syria was "liberated".
He instead proposed deferring to a legal council made up of Muslim clerics and thinkers, who would "put in place an appropriate plan for running the country, which of course will be based on Sharia, ruling on the basis of God's law".
Julani also vowed to punish those who followed "takfiri" doctrine, which holds that Muslim society has reverted to a state of unbelief, legitimising rebellion against the state and attacks on Muslims.
Recently, ISIS fighters have been filmed executing Muslims that they have declared as apostates.
"We strongly condemn those who go to extremes in declaring takfir of individuals or a general group of people. We consider all Muslim societies to be Muslim, and we consider Syrian society in general to be Muslim. We reject those who say that this society is an apostate one," he said.