As Syria’s revolution drags into its second year amid few signs that a U.N.-mandated cease-fire plan will end the violence, evidence is mounting that Islamist extremists are seeking to commandeer what began as a non-ideological uprising aimed at securing greater political freedom.

Activists and rebel soldiers based inside Syria say a small but growing number of Islamist radicals affiliated with global jihadi movements have been arriving in opposition strongholds in recent weeks and attempting to rally support among disaffected residents.

Western diplomats say they have tracked a steady trickle of jihadists flowing into Syria from Iraq, and Jordan’s government last week detained at least four alleged Jordanian militants accused of trying to sneak into Syria to join the revolutionaries.

A previously unknown group calling itself the al-Nusra Front has asserted responsibility for bombings in the cities of Damascus and Aleppo using language and imagery reminiscent of the statements and videos put out by al-Qaeda-affiliated organizations in Iraq, although no evidence of the group’s existence has surfaced other than the videos and statements it has posted on the Internet.

Syrian activists and Western officials say the militants appear to be making little headway in recruiting supporters within the ranks of the still largely secular protest movement, whose unifying goal is the ouster of the regime led by President Bashar al-Assad.

But if the United Nations’ peace plan fails to end the government’s bloody crackdown and promises of Western and Arab help for the rebel Free Syrian Army do not materialize, activists and analysts say, there is a real risk that frustrated members of the opposition will be driven toward extremism, adding a dangerous dimension to a revolt that is threatening to destabilize a wide arc of territory across the Middle East.

“The world doing nothing opens the door for jihadis,” said Lt. Abdullah al-Awdi, a Free Syrian Army commander who defected from the regular army in the summer and was interviewed during a visit he made to Turkey. He says that he has rebuffed several offers of help from militant groups in the form of arms and money and that he fears the extremists’ influence will grow.

“This is not a reason for the international community to be silent about Syria. It should be a reason for them to do something,” Awdi said.

Flow of jihadis reported

U.S. officials and Western diplomats in the region, who spoke on the condition of anonymity because of the sensitivity of the subject, say they have seen several indications that al-Qaeda-like groups are trying to inject themselves into the Syrian revolution, although they stress that the Islamist radicals’ impact has been limited. Al-Qaeda leader Ayman al-Zawahiri called on “mujaheddin” to head to Syria in support of the rebels earlier this year, and Western diplomats are convinced that operatives affiliated with al-Qaeda carried out a string of bombings in Damascus and Aleppo between December and March.

The diplomats say dozens of jihadis have been detected crossing the border from Iraq into Syria, some of them Syrians who had previously volunteered to fight in Iraq and others Iraqi. There may also be other foreign nationals among them, reversing the journey they took into Iraq years ago when jihadis flowed across the border to fight the now-departed Americans.

The Syrian government facilitated the flow of foreign fighters into Iraq for many years, and there are widespread suspicions that it may be covertly reactivating some of those networks to discredit the revolutionaries, deter international support for the opposition and create conditions under which the harsh crackdown by authorities will appear justified.

The regime portrayed the uprising as the work of radical Islamists in its earliest days, and the reports that extremists are surfacing in Syria only play into the official narrative, said Salman Shaikh, director of the Brookings Doha Center in Qatar.

“This drip, drip, drip of extremists across the border . . . there are signs the regime is aiding and abetting it,” Shaikh said. “And it will become a self-fulfilling prophecy.”

It is also plausible that these groups, adherents of a radicalized form of Sunni Islam, have turned against their former benefactors and are making their way back to Syria motivated by religious and sectarian zeal. Although many Syrian opposition activists insist that their revolution is not sectarian, a majority of Syrians are Sunnis, while Assad, along with most leading figures in the regime and in the security forces, belongs to the Shiite-affiliated Alawite minority, lending a sectarian dimension to the populist revolt.

Syrian activists and rebels insist that the extremists are not welcome in communities that have long prided themselves on their tolerance of the religious minorities in their midst, including Christians, Alawites, Druze, Kurds and Ismaili Shiites.

A rebel leader in northern Syria who asked to be identified by his nom de guerre, Abu Mustafa, described how he and his men drove out a group of about 15 radicals, all of them Syrian but none of them local, who arrived in a northern village in January. Led by a commander who identified himself as Abu Sulaiman, the group tried to recruit supporters for an assault on the nearby town of Jisr al-Shughour.

Abu Sulaiman “had money, he had weapons, and he sent a guy to negotiate with me, but I refused,” Abu Mustafa recalled in an interview in Turkey. “We asked him to leave, but he didn’t, so we attacked him. We killed two of them, and one of our men was injured. Then he left, but I don’t know where he went.”

“The good thing is that Syrians are against giving our country to radicals,” Abu Mustafa added. “But these groups have supporters who are very rich, and if our revolution continues like this, without hope and without result, they will gain influence on the ground.”

A largely secular revolt

There is a distinction between the naturally conservative religiosity of Syrians who come from traditional communities and the radicalism of those associated with the global jihadi movement, said Joseph Holliday, who is researching the Free Syrian Army at the Institute for the Study of War in Washington and believes extremists are a small minority.

“While there are elements [in the opposition] that are very conservative, they are not the driving force,” he said. “There is definitely an argument to be made that this will increase over time, because insurgencies often become more extremist over time, but for now the driving force behind this revolution is secular.”

Adherents of the strict Salafi school of Islam have emerged in many Syrian communities and are playing a role in the opposition, but they, too, are to be distinguished from the jihadis, said ­Yezid Sayigh of the Carnegie Middle East Center in Beirut.

“People who are local and pious and moving in an Islamist direction and are taking up guns don’t have the same organization and are not necessarily the same thing as jihadists, who are not necessarily al-Qaeda,” he said. “There’s a range of different directions and trends.”

Many activists fear, however, that the influence of the extremists is growing as Syrian rebels who have for months appealed in vain for Western military intervention look for help elsewhere.

“Of course it is growing, because no one is doing anything to stop it,” said a Syrian activist who spoke on the condition of anonymity because he fears retribution from some of the radicals he has encountered while attempting to organize the opposition in many northern communities.

“They have rules,” he said. “They say: If we give you money, you have to obey our orders and accept our leadership. Some of my friends drink alcohol, and they aren’t like this. But when they find no other way to cover their expenses, they join these groups and then they follow them.”

Special correspondent Ranya Kadri in Amman, Jordan, contributed to this report.