The small group of Free Syrian Army opposition fighters marched under a hazy sky outside Aleppo, raising their guns in the air, days after the government's Sheik Suleiman base fell into rebel hands.
"We are coming for you," they chanted in a video posted to the Internet, an apparent warning to Syrian President Bashar al-Assad. "God damn your soul, We are coming for you."
But the week's major victory at the military base was not led by the Free Syrian Army. The Syrian Observatory for Human Rights says it was led by Islamist fighters with the al-Nusrah front, giving them access to tanks, missiles and other heavy weaponry.
“We see a shift toward Islamicization of the ideology of the people who are fighting on the ground, which is a concern,” says Mattaz Suheil with the Britain-based Observatory. “They’re more cohesive as a group and that’s a problem because it’s radicalizing young people who want to be part of a military campaign against the regime, against a minority in Syria and they eventually adopt the more radical view.”
Suheil says the influence of groups like al-Nusrah is having an impact that goes beyond the battlefield.
“We see in Aleppo city now they’ve installed an Islamic court and a police of vice and virtue which means people, civilians, are being tried for moral crimes.”
West taking notice
Concern has escalated in the West. The U.S. Treasury on Tuesday sanctioned two senior leaders of the al-Nusrah front for acting on behalf of al-Qaida in Iraq, while the U.S. State Department designated al-Nusrah a terrorist organization.
"This, again, goes to the environment that Assad and his regime have created with their violence - that they have, as we have been concerned about for many months, created an environment with this violence which extremists can now try to exploit," State Department spokeswoman Victoria Nuland told reporters at a recent briefing.
Activists and some experts warn the problem runs even deeper.
Rohan Gunaratna heads the International Center for Political Violence and Terrorism Research at Nanyang Technological University in Singapore and has studied al-Qaida extensively. He warns al-Nusrah is positioning itself well.
"Al-Nusrah has emerged as the most capable and most lethal enemy of the Syrian government," Gunaratna says, adding the Free Syrian Army "does not have the ability to match or compete against al-Nusrah."
He says if and when the government of Bashar al-Assad tumbles, it will be al-Nusrah and other like-minded groups competing for power.
Other experts and activists are hesitant to make such bold predictions but point out al-Nusrah is better organized than most of the other forces working to topple the Syrian government.
They say the group draws on support from al-Qaida and on funding from private donors - which has allowed it to arm with better weapons. Making al-Nusrah even more dangerous are the lessons the group's leaders have learned from Iraq.
Stratfor, a private intelligence firm, says for months al-Nusrah has been trying to win over the local population instead of alienating them with attacks that cause mass casualties, a tactic that in Iraq led to the rise of the counter-jihadist Sunni Awakening Councils.
Al-Nusrah also has been willing to work with other groups, like the Free Syrian Army, to achieve the opposition's main objective of toppling the Assad regime.
While concern appears to be growing, not everyone is convinced al-Nusrah and other Islamist groups represent a major threat.
“In a post-Assad era is Syria going to be Islamist? I think that is a largely exaggerated fear,” says Leila Hilal, director of the Middle East Task Force at the New America Foundation.
Hilal says al-Nusrah's leadership may subscribe to a hardline Islamist doctrine but that the make-up of its fighters is much more diverse.
“People often move from one group to another depending on the availability of resources, depending on operational decisions that may be made,” she says.
Some analysts say what happens next depends not necessarily on groups like al-Nusrah, but on what actions are taken by the U.S. and other key players in the international community.
“Syria has not been a fertile environment for al-Qaida," says Salman Shaikh, director of the Brookings Doha Center. "But as long as we’ve allowed the situation inside Syria to drift and the power and security vacuum that comes with that, al-Nusrah has gained ground and will continue to gain ground.”
Shaikh says al-Nusrah - and al-Qaida - can still be "put back in their place” - though it will require a more effective opposition coalition and help from the U.S.
“I still believe that a robust U.S. role in the train and assist aspect of the rebels' military effort is extremely important," he says. "It will also, I may add, lay the groundwork for the kind of more national forces we will need in Syria to ensure a safe and secure environment for a political transition.”
In the meantime, Shaikh and others fear many Syrians will act out of desperation and take help from wherever they can get it.
There is also the question of aid coming from Gulf Arab states like Saudi Arabia and the effect it is having.
“The evidence so far has pointed to them actually contributing to a certain amount of fragmentation where weapons have gone to different groups in a less controlled and considered fashion than they could have been,” Shaikh says.
There is concern that such fragmentation is doing more than allowing Islamist groups to strengthen their positions. Some analysts say combined with a weak opposition coalition, the situation could give rise to sectarian strife.
“It’s a real race against time, I would say, before we see almost a free-for-all inside Syria,” Shaikh says.
For now, the Syrian Observatory's Mattaz Suheil worries about the radical Islamists and the fight their growing victories are enabling them to lead.
"The conflict and the form of the military conflict is going to intensify with higher caliber weapons being used by both sides which by their nature are more indiscriminate,” he says.
“What we’re seeing on the ground are not gains. We’re seeing the largest city in Syria - and its economic hub - turned into a battle zone. The infrastructure of the state is completely destroyed."