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

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.