ALEPPO, Syria — Justice was swift and brutal when fighters of the Al Nusra Front militia caught a man accused of raping and killing a young girl in front of her father. They beheaded the man and left his body in the street.

The presence of women and children didn’t deter them. Neither did the appeals of other rebels at the checkpoint in the embattled neighborhood of Salahuddin.

Members of the Free Syrian Army, the main rebel force, said that the man was a member of a pro-government militia and that they had no doubt he was guilty. They also had no objection to killing him, but they did object to a public beheading.

“They don’t even shoot them, they slaughter them with a knife,” said Hassan, a rebel fighter nicknamed the “Barber of the Revolution” because of his previous occupation. “These aren’t Islamic manners.”

The appearance of Al Nusra Front, which is made up mostly of Syrians and some say has ties to Al Qaeda, and the presence of a small contingent of foreign fighters threatens to complicate the uprising against President Bashar Assad, a conflict that already has taken on sectarian overtones.

For months as Assad’s forces increasingly unleashed tanks, helicopters and fighter jets on towns and cities, the international community kept the rebels at arm’s length, encouraging diplomacy and a small amount of military and other aid. But that appeared to make it more likely that Al Qaeda or a similar group would move into the vacuum.

A NATO air campaign helped Libyans topple Moammar Kadafi last year, but as the prospect for similar intervention in Syria dimmed, some openly said they would welcome a group like Al Qaeda to help them get rid of Assad. Fighters who witnessed and later described the beheading made it clear that they welcome help from any quarter but that they were uneasy with some of Al Nusra Front’s methods.

International concern about such groups rose after twin car bombings in May killed 55 near a military intelligence complex in Damascus, the capital. The Free Syrian Army disavowed the attack. There were conflicting reports about whether Al Nusra Front had claimed responsibility.

U.N. Secretary-General Ban Ki-moon said he believed that Al Qaeda must have been behind it. “This has created again very serious problems,” he said.

“The Syrian conflict serves as a magnet for these groups,” said Andrew Tabler, senior fellow at the Washington Institute for Near East Policy who is in Antakya, Turkey, near the Syrian border, meeting with opposition figures. “But, and this is important, they’re still an extremely small part of the groups that are fighting the Syrian regime.”

As the conflict drags on, though, their number is likely to grow, he said.

Tabler described Al Nusra Front as “an Al Qaeda-affiliated group” that is recognized by other such groups and mentioned in their online forums.

Whether or not they are connected to larger groups, the conflict between mostly Sunni fighters and Assad’s regime, which is dominated by Alawites, an offshoot of Shiite Islam, has lured some foreign Islamic militants who see it as an obligation to help oppressed Sunni Muslims.

“They are concerned with lifting the oppression,” said Majid, one of the fighters near the front line in Salahuddin. “They saw what was happening here and they came to fight with us. They don’t have any plans beyond that.”

But rebel fighters agree that most of Al Nusra Front’s members are Syrians. One of the group’s leaders, Abu Khalid, from another Aleppo neighborhood, only recently began carrying a rifle and spends most of his time preaching to the rebels at the front lines.

Abdulaziz Salameh, who heads Al Tawheed Brigade, which includes most of the militias fighting in Aleppo and its suburbs, estimated that there were fewer than two dozen foreigners among them, mostly from Libya, Tunisia and Chechnya.

Observers have pointed to the increasing prevalence of the black flag with the Muslim declaration of faith written in white, which is used by Al Qaeda, as a sign that the group has infiltrated the Syrian conflict.

Rebels said this flag, used by the prophet Muhammad during wars in the 7th century, doesn’t belong to any one group.

“They raised a flag that is black that says, ‘There is no God but Allah,’ and they said they are Al Qaeda. We raised the same flag and we are not Al Qaeda,” Salameh said. “I sat with them and many smoke like me, they are not extreme. I found their behavior to be acceptable and moderate.”

During the Muslim holy month of Ramadan, many members of Al Nusra Front, like most of the other fighters on the front lines, took an exemption from the religious obligation to fast during the day.

Although his group didn’t condone the executions in the streets, which are said to be rare, Salameh said it was not easy to control the behavior of people whose family members have been killed and are seeking revenge.

“We tell them we have prisons and people to interrogate them,” he said.

There are smaller indications of their doctrine.

One night at a school in Salahuddin that served as both a field hospital and prison, shelling thundered outside.

A teenager walked in and complained that members of Al Nusra Front had demanded that he stop wearing his shirt, which had a cartoon of a person on the front. Drawings of living things are generally disavowed in Islam.

“It’s none of their business,” he said.

Others have reported that members of the group have demanded that women wear the hijab.

“It’s a bit hard to say what their long-term aspirations are,” Tabler said. “But in other countries, especially where you don’t have a centralized state power, they could just set up shop. I think that’s the real worry.”

But few in Syria seem concerned about the long-term ramifications of foreign or fundamentalist elements among the rebels.

Some believe that it should become the government’s role to enforce morality. Others, including some members of minorities that elsewhere have been persecuted by Al Qaeda-related groups, contend that Syria’s moderate and pluralistic society would never accept a religious government.

“I don’t worry about an Islamic state because the Syrian bourgeoisie won’t allow it,” said Marcell Shehwaro, a well-known Christian activist in Aleppo. “The Muslims in Syria have a different understanding, which doesn’t include forcing the hijab.”

Despite the divergent paths of their uprisings, Shehwaro said, she envisioned Syria’s future to be like that of Libya, which recently elected a liberal Muslim president.

“In the end the Syrians are going to choose who feeds them and gives them dignity, not who says God is great,” she said.

Hassan said the Islamist fighters won’t have a role in a post-Assad Syria.

“After the revolution we won’t let them stay,” he said. “When we’re done they will go to another country to fight. Wherever they can find jihad they go.”