WASHINGTON — Al-Qaida has advanced beyond isolated pockets of activity in Syria and now is building a network of well-organized cells, according to U.S. intelligence officials, who fear the terrorists could be on the verge of establishing an Iraq-like foothold that would be hard to defeat if rebels eventually oust President Bashar Assad.

At least a couple of hundred al-Qaida-linked militants are already operating in Syria, and their ranks are growing as foreign fighters stream into the Arab country daily, current and former U.S. intelligence officials say. The units are spreading from city to city, with veterans of the Iraq insurgency employing their expertise in bomb-building to carry out more than two dozen attacks so far. Others are using their experience in coordinating small units of fighters in Afghanistan to win new followers.

In Syria on Friday, rebel commanders appealed anew for new and better weapons from abroad, complaining that Assad’s forces have them badly outgunned from the air and on the ground. In fact, rebel leaders say that with so little aid coming to them from the U.S. and other nations, they are slowly losing the battle for influence against hardline militants. They say their fighters are sometimes siding with extremists who are better funded and armed so they can fight the far stronger Syrian army.

It all could point to a widening danger posed by extremists who have joined rebels fighting the Assad government. Although the extremists are ostensibly on the same side as Washington by opposing Assad, U.S. officials fear their presence could fundamentally reshape what began as a protest movement for reform composed of largely moderate or secular Syrians. The opposition expanded into a civil war pitting Assad’s four-decade dictatorship against a movement promising a new, democratic future for the country.

The intelligence also offers some explanation for the Obama administration’s reluctance to offer military aid to the anti-Assad insurgency, which Washington says it is still trying to better understand. U.S. officials have repeatedly rejected providing any lethal assistance to the conflict that has killed at least 19,000 people over the past 17 months. With the U.S. weighing its options, Secretary of State Hillary Rodham Clinton will discuss the situation with top Turkish officials and Syrian opposition activists in Istanbul on Saturday.

Officials described the intelligence on condition of anonymity because they were not authorized to publicly discuss confidential internal talks among intelligence and administration officials

Underscoring the administration’s desire to step up efforts against the Assad government without providing weapons, the U.S. set largely symbolic sanctions Friday on Syria’s state-run oil company and Iranian-backed Hezbollah. It accused Iran and the Lebanese Shiite militant group of helping prop up Assad.

Neither action will mean much immediately. Americans have been banned from doing business with Hezbollah since the U.S. declared it a foreign terrorist organization in the 1990s. Decades of U.S. sanctions against Syria have hampered energy trade between the two countries, and President Barack Obama blacklisted any new imports a year ago.

Meanwhile, Syrian rebels were running low on ammunition and guns Friday and appealed for international help as government forces tried to consolidate their control over Aleppo, the country’s largest city and a deadly battleground in recent weeks.

“The warplanes and helicopters are killing us, they’re up there in the sky 15 hours a day,” said Mohammad al-Hassan, an activist in Aleppo’s main rebel stronghold of Salaheddine. “I don’t know how long this situation can be sustained.”

As for a possible diplomatic solution, former Algerian foreign affairs minister and longtime U.N. official Lakhdar Brahimi emerged as a candidate to replace Kofi Annan as peace envoy to Syria. Annan announced his resignation last week, ending a six-month effort that failed to achieve even a temporary cease-fire as the country descended into civil war.

A fresh wave of civilians was streaming across the border into neighboring Turkey. Officials there said more than 1,500 Syrians had arrived over the previous 24 hours, increasing the number of refugees in Turkey to about 51,500.

In Syria, Assad, a member of the country’s Alawite minority, has blamed the uprising against him on Sunni terrorists and the West. American officials say the claims are only an excuse for brutal tactics of repression as part of a desperate attempt to hold onto power. But they concede that the extremist presence in Syria is growing.

U.S. officials say the number of al-Qaida operatives remains small in the context of the larger anti-government insurgency, with perhaps only 200 or so who are active. But ranks are growing, the officials said.

Once operating as disparate, disconnected units, the al-Qaida cells are now communicating and sometimes cooperating on missions, with a command-and-control structure evolving to match more sophisticated operations in places like Iraq and Afghanistan, U.S. officials said. The coordination is sometimes as good as that of Syria’s mainstream rebels.

“There is a larger group of foreign fighters ... who are either in or headed to Syria,” the State Department’s counterterrorism coordinator, Daniel Benjamin, told reporters recently. He said Syrian opposition groups “assured us that they are being vigilant and want nothing to do with al-Qaida or with violent extremists.”

Still, the administration clearly has reservations. Speaking earlier this week, Clinton stressed a need for Syrians to avoid sectarian warfare when the Assad government falls, as the U.S. insists will happen.

“Those who are attempting to exploit the misery of the Syrian people, either by sending in proxies or sending in terrorist fighters, must recognize that that will not be tolerated, first and foremost by the Syrian people,” she said.

But the Brookings Institute’s Bruce Riedel said such U.S. pronouncements are having limited effect.

“Clinton is going to tell them ‘clean up your act or we can’t help you,”’ said Riedel, a former adviser to the Obama White House. “The rebels are saying, ‘You aren’t helping us anyway.”’

The administration says it is providing $25 million in nonlethal aid, primarily communications, to the Syrian opposition. The rebels have gotten their weapons through army defectors, looted government depots, the black market and the assistance of Sunni governments such as Saudi Arabia and Qatar. The U.S. fears weapons ending up in extremists’ hands.

But Syrian rebel commanders complain that their fighters are attracted to join up with better-armed extremists.

The extremists “come with weapons and money,” said Murhaf Jouejati, a professor at the National Defense University and a member of the opposition Syrian National Council. Their weapons include mortars, anti-tank weapons and rocket propelled grenades, many left over from old Iraqi army stockpiles, he said. They have cash thanks to donations from hardline sympathizers throughout the region who see Assad’s crackdown as an attack on Syria’s Sunni majority.

The extremist influence in Syria is debated, however, within the U.S. government. Some deem it minimal or ad hoc, and one official insisted there is no sign al-Qaida is “influencing command-level decisions” by rebel forces.

Rand analyst Seth Jones said the presence of extremists was small but growing. He said the U.S. should consider using its forces or getting the rebels or a regional proxy to attack the al-Qaida units.

“There has been talk that some operatives in Pakistan are saying, ‘Why don’t we see if we can make it to Syria,”’ he said. “That’s where the fight is.”

If they win local loyalty by fighting alongside Syrian rebels, they will be hard to eliminate no matter how Syria’s future pans out, said former CIA analyst Riedel.

“Look at Iraq, where we decimated them time and again,” Riedel said. “They’re still there.”