BEIRUT, Lebanon — As a government warplane soared over the northern Syrian city of Raqqa recently, a fighter from the Islamic State of Iraq and Syria, the country's most radical group linked to Al Qaeda, watched from behind an antiaircraft gun mounted on a pickup truck. Fighters and activists from rival insurgent factions urged him to fire. He did not.
The others were incredulous, recalled one, who supports the Nusra Front, a rival group that has Al Qaeda's official stamp of approval as its representative in the fight against President Bashar al-Assad. But the man on the truck replied, "We are here to establish the Islamic state, not to fight Assad."
Such disputes helped set off the infighting that has swept insurgent-held northern Syria for the past week, leaving more than 500 dead, as a broad array of factions have turned against the Islamic State of Iraq and Syria, known as ISIS, in a showdown over tactics, power and ideology within a Sunni jihadist movement that has drawn fighters from across the world.
The dispute has reverberated far beyond Syria's borders, analysts say, for instance carving the same divisions in the jihadist movement in Egypt between pragmatists and ideological purists from the austere Salafist movement.
In some places, like Raqqa, the fighting has even pitted the two Qaeda-inspired groups against each other. Nusra's leaders and supporters accuse ISIS of the grave error of focusing too soon on building a radical Islamic state, the ultimate goal for both groups, at the expense of the war against Mr. Assad. In the process, they say, it has alienated potential allies, civilians and militants alike.
While many here see the fighting as a wrestling match over the future character of the rebel opposition, experts say it has many other facets.
"It is a struggle for the heart and soul of jihadism, but it's also a dirty little turf war," said Aron Lund, who studies militant Islam and edits the Carnegie Endowment's Syria In Crisis website.
At stake is power in the insurgency and over daily life in rebel-held northern Syria, where ISIS has sought to impose draconian religious rules and clamp down on public protest. The outcome is unclear, and different factions are fighting for different scenarios, whether to eradicate ISIS or bring it into the fold as more a cooperative participant in the insurgency.
A decisive victory over ISIS could lift the Syrian opposition ahead of planned peace talks in Switzerland on Jan. 22, if disparate factions show a new unity and coordination that they could use to regain momentum against the government. The timing has prompted speculation that the coalition of rebel leaders in exile, which has steadily been losing influence, or its regional and Western backers have encouraged or supported the fighting.
The exile coalition hopes the battles will persuade the West that the insurgents, not Mr. Assad, can best stop Al Qaeda from establishing a base in the country. That security threat has made foreign powers, not to mention many Syrians, more open to accommodation with the government.
Advocates of increased Western aid to the rebels argue that the fighting shows that the Islamic Front, a collection of Islamist Syrian groups now fighting ISIS, is not a Qaeda-like threat. But many of its leaders, and even some in other groups that nominally answer to the exile coalition, say they have no quarrel with the main Qaeda affiliate, the Nusra Front.
Nusra, indeed, could be the main beneficiary of the scuffling. Taking a more pragmatic approach has helped Nusra recruit many Syrian fighters and could now enable it to cement alliances, rid itself of a rival for Qaeda-inspired donations, absorb foreign fighters fleeing ISIS and embed itself more deeply in society. In trying to mediate the dispute, it has also set itself up as peacemaker and power broker.
But Nusra has come in for its own share of criticism for clashes with rebel groups, killing civilians and branding minority Shiites and Alawites as the enemy. For the West and for secular Syrians, said Michael Hanna, an analyst at the Century Foundation, "Nusra, because they are more pragmatic, are a much more intractable problem.
The picture is further complicated by the fluidity of alliances. In the highly localized conflict, ISIS and its rivals remain united even now against Kurdish militias in the northeast. Fighters often switch factions, not always sharing the views of nominal leaders, and sometimes Qaeda trappings signify branding more than chain of command.
Some homegrown rebels say they are fighting ISIS to reclaim the insurgency from extremists and regain trust from supporters who want a pluralistic Syria. Others say they want to salvage an Islamist movement stained by the extreme brutality of ISIS and its zeal for attacking fellow Sunnis.
"They call themselves Qaeda because they know the young fighters love our great sheikh Osama bin Laden," said Abu Ibrahim al-Masri, an Egyptian fighter near the central city of Hama. He spoke in a video announcing that he was defecting from ISIS to Nusra because he had been ordered to kill civilians and members of the rebel Free Syrian Army instead of army soldiers and, as he put it, Alawites.
"We came to kill them, not to kill our brothers from the F.S.A.," he said.
Those sentiments fail to reassure Syria's religious minorities and others opposed to Islamic rule in a country long steeped in secular Arab nationalism. Syrian Muslims who are personally conservative tend to be more traditional than ideological and open to religious coexistence, though more and more are being radicalized during the nearly three-year war.
While the Nusra Front has rejected parliamentary democracy, it has also opposed the autocracy of ISIS in favor of a consultative approach to decision-making and for putting off politics until after the war, said Aymenn Jawad al-Tamimi, a fellow at the Middle East Forum.
The fight mirrors, in some ways, a demand of the original uprisings in 2011 for accountability from the powerful. A broad array of insurgents say they are simply fed up with the arrogance and abuses of a foreign-led faction that came late to the war, took over territory others had wrested from the government and posted street signs declaring Syrian towns part of its Islamic state. It provided some social services, but also arrested and executed hundreds of antigovernment fighters and civilians, and even used suicide bombs against other Islamist militants.
On Friday in the northern city of Aleppo, protesters in a district called Al Bab waved the green, white and black flag of the uprising, often replaced nowadays with the black flag of jihad. In a scene reminiscent of early protests against Mr. Assad, they chanted, "Al Bab is free, ISIS should go. The traitor is the one who kills his own people."
The next evening, people rushed to find food, even scarcer than usual because of roads blocked by new fighting, and get home by sunset. A civilian antigovernment activist, who gave only a first name, Shadi, said he initially welcomed ISIS, thinking they would be fierce fighters, but turned against them after they executed a young boy they accused of insulting the Prophet Muhammad.
"We expected the ISIS fighters to liberate the western part of the city," he said, "not deploy their fighters in the liberated areas and apply their religious rules on people."
But another Aleppo activist, Thaer, worried that without the iron rule of ISIS, chaos would worsen. "It will be like Chicago," he said, in the days of Al Capone.
A Syrian ISIS fighter, Abu Omar, recuperating from an injury in his family home nearby, said that the group would keep attacking its rivals. They were tools of the West, he said. "It is an American order to finish us."