ALEPPO, Syria — A shadowy jihadist organization that first surfaced on the Internet to assert responsibility for suicide bombings in Aleppo and Damascus has stepped out of the shadows and onto the front lines of the war for Syria’s cities.

Here in Aleppo, the al-Nusra Front for the Protection of the People of the Levant, widely known as the Jabhat al-Nusra, is fielding scores of fighters, some of them foreigners, in the battle for control of Syria’s commercial capital, a key prize in the bitter war of attrition being waged across the country.

The group, suspected of affiliations to al-Qaeda, says it is also fighting in other locations, including the cities of Homs and Idlib and the suburbs of the capital, Damascus. Its growing role has prompted concerns that the 17-month-old uprising against Syrian President Bashar al-Assad’s regime is becoming radicalized as the bloodshed soars.

On a recent morning, three jihadist fighters chambered rounds into their AK-47 rifles as their bearded driver sped through Aleppo’s streets in a bullet-ridden white van.

“If shooting starts, put your head down,” one of the jihadists said as the van headed toward the flashpoint Salahuddin neighborhood, blending in with the battle-scarred vehicles of other fighters hurtling through the streets.

Jabhat al-Nusra’s growing visibility on the streets of Syrian cities highlights one of the reasons the United States and its allies have been reluctant to arm Syrian rebels even as Obama administration officials repeatedly insist that Assad must go. Fears are widespread among Western governments that weapons sent to the rebels could wind up in the hands of extremists and be turned against their benefactors in a region already taut with sectarian and geopolitical rivalries.

In an interview at the mosque that serves as his headquarters in the Shaar neighborhood of Aleppo, Jabhat al-Nusra commander Abu Ibrahim said he has 300 men under his control. About 50 of his fighters were seen milling around the mosque, many wearing the baggy, calf-length pants and long beards associated with devout Islamists. Others were inside.

Most of those fighting for Abu Ibrahim, a 32-year-old stone mason from a nearby village, are Syrians from Aleppo and the surrounding countryside. But some are Arab volunteers, among hundreds from the region and beyond who are thought to have trickled into Syria in recent months to join the fight against Assad’s regime. Abu Ibrahim said his contingent included men from Morocco, Libya, Tunisia and Lebanon, as well as one Syrian who had fought in Iraq against the Americans.

‘They fight without fear’

Jabhat al-Nusra is the only Syrian rebel group that posts on a Web forum that is used by al-Qaeda chief Ayman al-Zawahiri and known affiliates of the terrorist network. This suggests a link, at least through its media department, to the main al-Qaeda organization, a connection that endows Jabhat al-Nusra with a credibility among jihadists that other groups lack, said Aaron Y. Zelin, a fellow at the Washington Institute for Near East Policy.