The conflict over the town, Azaz, has shuttered a Turkish border crossing long used to supply the rebel movement and heightened tensions between rebels who seek the ouster of President Bashar al-Assad and extremists who want to erase Syria’s borders and establish a transnational Islamic state.

The Qaeda group, the Islamic State in Iraq and Syria, known as ISIS, routed local rebels to take control of Azaz two weeks ago and has since set up checkpoints around the town and taken over the bases of other rebel groups.

Rebels who oppose the ISIS jihadists have collected their forces at the Bab al-Salameh border crossing a few miles away and are preparing to protect it should the jihadists advance.

Turkey has kept the crossing closed since Sept. 19 because of security concerns, a Turkish Foreign Ministry official said.

Seeking to end the crisis, a group of six powerful rebel brigades released a statement late Wednesday calling for an immediate cease-fire.

In a jab at the strict ideology of the ISIS jihadists, the statement told them not “to shed the blood of Muslims and be hasty in calling them heretics and apostates.” It also called on both sides to submit themselves to the Shariah Commission, a rebel-run court in the northern city of Aleppo, within 48 hours to resolve the problem.

It was unclear if the ISIS fighters would heed the call.

The rise of ISIS in rebel-held areas in northern and eastern Syria has posed a problem for the broader rebel movement. While many insurgents are deeply Islamist themselves, their focus remains on toppling Mr. Assad, and they accuse ISIS of prioritizing its own jihadist agenda over the fight against the president. But the rebels hesitate to confront ISIS, saying their resources are already stretched by fighting the government.

ISIS seized Azaz from the local rebel group known as the Northern Storm that led the fight last year to oust government forces from the town.

A Northern Storm commander reached by telephone said that since taking over the town, ISIS had attacked his group’s bases in nearby villages and that his fighters were shooting back with heavy machine guns meant to down airplanes.

“This is all we can do until we find a way to end this,” said the commander, who goes by the name Abu Yamen.

This week, a Qaeda spokesman accused the Northern Storm of attacking first and said the rebel group had struck a deal with Senator John McCain during his brief visit to Syria this year to fight against ISIS “and hit the mujahedeen.”

Turkey’s Parliament on Thursday extended a mandate for the army to launch military operations in Syria if necessary, as the government argued that the use of chemical weapons by forces loyal to the Assad government had aggravated Turkey’s national security concerns.

Turkey’s president, Abdullah Gul, speaking to reporters on his way to New York last week to attend the opening session of the United Nations General Assembly, called radical Islamist groups in Syria a serious security threat for his country, and warned that the continuing civil war there “could produce an Afghanistan in Eastern Mediterranean,” according to the daily newspaper Hurriyet.