ISTANBUL, Turkey (CNN) -- A year ago, Ibrahim Qobani was an idealistic 19-year-old revolutionary who sang about freedom.
Always dressed in a scarf and fingerless gloves woven with the colors of the Syrian rebel flag, Qobani worked with a team of pro-democracy activists in Syria's northern Idlib province. He would sing from the rooftops during boisterous anti-government protests, complete with humorous animations that begged the international community to help stop Syrian government atrocities.
But today, Qobani appears much different in a series of YouTube videos.
Gone is the scarf with colors of the rebel flag. Instead of leading the chants, the young man stands in the crowd smiling as one man sings, "We destroyed America with a civilian plane, turned the World Trade Center into a pile of dirt. If you call me a terrorist, I say it's an honor."
In another video, Qobani stands cheering with a crowd of bearded men as a little boy sings, "Our commander is Bin Laden. He showed the Americans the strength of our faith." A man gives the boy a knife, which he proceeds to slice through the air as he sings, "Our police is Nusra. Just wait Alawites. We will come to slaughter you."
It was a chilling warning to the minority religious sect of Syrian President Bashar al-Assad, who has stacked the top ranks of his security forces with fellow Alawites. Qobani declined several requests to speak with CNN. Several of his friends, however, confirmed that he is now a member of Jabhat al-Nusra, or the Nusra Front, the most famous of the hard-line Islamist rebel groups now fighting against the Syrian regime.
Qobani's ideological evolution is symbolic of a broader shift that many Syrians say they're seeing nearly two years after the anti-government uprising began.
"There is an increasing militarization and now increased radicalization of the revolution," said Rafif Jouejati. She is the English spokeswoman for the Local Coordination Committees in Syria, one of the first groups to organize peaceful anti-government protests in the spring of 2011.
"Jabhat al-Nusra continues to make gains," Jouejati said. "They continue to increase in popularity, particularly as they begin to implement social services."
In the last year, the Nusra Front has grown from a shadowy group claiming responsibility for deadly car bombs to what Syrians describe as a highly disciplined fighting force that continues to attract recruits from the more secular Free Syrian Army rebels.
Nusra Front fighters are said to be leading the ongoing siege of the Khweiris military airbase in Aleppo province. They are also credited with helping lead the capture of the Taftanaz helicopter airbase in Idlib province last month.
The high-profile Islamist victories on the battlefield have been accompanied by another trend. Gradually, black Islamist banners have replaced the distinctive green, black, white and red flags of the Syrian rebels at weekly anti-government protests.
"After two years of killings and butchering and the entire world standing by and watching us, now we depend on God only. So we started raising the banner of Shahada, the black banner of war," explained a Syrian activist who has spent much of the last two years organizing anti-government protests in Idlib province.
The Shahada refers to the Islamic creed "There is no god but God, Mohammed is the messenger of God." It is one of the most important pillars of Islam. It is also invoked for martyrdom on the battlefield.
After two years of killings and butchering and the entire world standing by and watching us, now we depend on God only.
The rapid rise of hard-line Sunni Muslim groups like the Nusra Front -- some of which have seen their ranks swelled by foreign jihadi fighters -- is a trend that makes Jouejati and other more secular revolutionaries deeply uncomfortable.
It is also making Washington uneasy. In December, the U.S. government blacklisted the Nusra Front, labeling it a terrorist organization.
"We blacklisted the Nusra Front because of its intimate links with al Qaeda in Iraq ... which is responsible for the killing of thousands of Iraqis and hundreds of Americans," said Robert Ford, the U.S. ambassador to Syria, in an interview with CNN last month. "We know what al Qaeda in Iraq did and is still doing, and we don't want it to start doing that in Syria."
But some Syrian opposition leaders blame Western inaction for the recent growth of the Islamist groups.
"The U.S. and the European Union didn't help us, and that created an increase in Islamic radicalism here," said Marwan Gayed in an interview in Aleppo last month. Gayed was a judge who defected from the Syrian government and helped launch the United Courts Council, an opposition-run court that is trying to institute law and order in rebel-controlled parts of Aleppo.
Like many in the opposition, Gayed has viewed Islamist groups like Nusra as uneasy partners in the campaign to overthrow the al-Assad regime.