Loveday Morris / The Washington Post

REYHANLI, Turkey — While the emergence of al-Qaida's Islamic State of Iraq and Syria as a major force in the Syrian civil war has caused deep concern for many rebels, one group's fighters claim its presence has given them a popularity boost.

Until ISIS asserted its place in the war earlier this year, Jabhat al-Nusra had the reputation of being the most radical wing of the opposition seeking to oust Syrian President Bashar Assad. It was the first to claim responsibility for car bombings against government targets and was quickly designated a terrorist group by the United States.

But the newcomer, with its high proportion of foreign fighters, has eclipsed Jabhat al-Nusra as it enforces bans on smoking, forces women to wear the veil, carries out public executions and clashes with other rebel groups in an attempt to gain control in opposition areas.

Amid concerns about ISIS's expansionist plans, other groups are looking to Jabhat al-Nusra as a counterbalance and have been teaming up with Jabhat al-Nusra on the battlefield.

Jabhat al-Nusra fighters say the group itself is changing, benefitted from a drift of its more extreme members to ISIS and helping the group to present itself as a more mainstream — and more Syrian — force.

That change comes amid an overall radicalization of the Syrian rebel movement and a weakening of moderate groups that has left the West wary of supplying support.

Abdul Kareem Dahneen, a 31-year-old from the northern city of Idlib who joined Jabhat al-Nusra a year ago, said the group's relations with Syrians have improved in recent months. He put that largely down to the departure of foreign fighters who travelled to Syria to fight for an Islamic caliphate and had different ambitions than the fighters who rose up to battle Assad.

“Of course this had an effect,” he explained. “Now with Jabhat we are more moderate with the people. The foreigners would see if you aren't wearing a veil they might threaten to kill you. We would explain why it was haram (forbidden) and say you should stop. You make a choice.”

When ISIS emerged as a force in March, all the foreign fighters in his unit — 30 out of 40 men, hailing from places such as Chechnya, Tunisia and Algeria — left to join the group, he said. They packed up and started a new base less than 100 yards down the road.

He said the shift in perception of Jabhat al-Nusra has helped make up for the drain in foreign fighters as Syrians who may have otherwise been dissuaded decided to join up.

Mohammed, a 25-year-old Jabhat al-Nusra fighter who would not give his last name, said he would have had reservations about joining the group before the foreigners left.

“The very extreme foreigners went to Islamic State and the Syrians stayed with Nusra,” he said. “We are Syrians. We refuse these extremists ways they are dealing with Syrians.”

Some rebel groups say they see Jabhat al-Nusra as key to curbing ISIS's expansion in rebel-held areas and are keen to reach out.

Once focused on solo operations or on cooperating with the hard-line Islamist battalions, Jabhat al-Nusra has been fighting alongside a much wider array of rebel groups in recent months.

“They have changed their strategy recently and become closer to the mainstream FSA,” said Yasser al-Haji, a spokesman for the Free Syrian Army's military council in Aleppo. “They are trying to improve their image a little bit. With the West not supporting us, we have no choice but to cooperate.”

“They can play a vital role,” said Rami Jarrah, a Syrian activist and co-director of the citizen journalist ANA New Media Association. ISIS attacked ANA's Raqqa office last month, and one of its employees was kidnapped. “We aren't going to be able to take on ISIS without Nusra. Them being part of the solution is not crazy to us.”