Rebel fighters walk holding their weapons to a position on a front-line in a rebel-held suburb of Damascus on August 23, 2014. (Rami Al-Sayed/AFP/Getty Images)

A U.S. offensive in Syria against the radical Islamist group that beheaded an American journalist would likely be constrained by persistent intelligence gaps and an inability to rely on fleets of armed drones that have served as the Obama administration’s signature weapon against terrorist networks elsewhere, U.S. officials said.

The Pentagon has conducted daily surveillance flights along Iraq’s border with Syria in recent weeks as part of a push to bolster U.S. intelligence on the Islamic State without crossing into Syrian airspace and risking the loss of aircraft to that nation’s air defenses, officials said.

The CIA has also expanded its network of informants inside Syria, largely by recruiting and vetting rebel fighters who have been trained and equipped at clandestine agency bases in Jordan over the past two years, U.S. officials said.

Still, senior U.S. intelligence and military officials — speaking on the condition of anonymity to discuss sensitive operations — said American spy agencies have not yet assembled the capabilities that would be needed to target Islamic State leaders and provide reliable-enough intelligence to sustain a campaign of strikes.

“Our intelligence is improving since we began devoting the resources to doing that, but we still have only modest visibility into what is going on in Syria,” said Rep. Adam B. Schiff (D-Calif.), a member of the House Intelligence Committee.

A senior U.S. intelligence official said that “it would probably take some number of months to really build up the necessary intelligence architecture” to expand the U.S. air campaign underway in Iraq against Islamic State positions in Syria. “This is not going to end anytime soon.”

The Obama administration has counted on Predator and Reaper aircraft to carry out hundreds of strikes against al-Qaeda targets in Pakistan and Yemen, countries where it has at least tacit permission to fly armed drones. The planes conduct near-constant surveillance over extensive territory in both countries and often spend days tracking targets before a missile launch.

But the use of drones is far riskier in Syria, where the forces of President Bashar al-Assad guard the country’s airspace with missile batteries and fighter aircraft. The Islamic State seeks to overthrow Assad, and strikes against the group would be in his interest. But allowing American drones to reach cities such as Raqqah — an Islamic State stronghold — would also probably be seen by Assad as a threat, in part because such aircraft could gather valuable intelligence on his forces.

U.S. officials stressed that Obama has not made a decision to launch strikes in Syria — an action the administration has avoided since the start of that country’s civil war. But the video-recorded execution of American journalist James Foley this past week has prompted a reevaluation of the threat posed by the Islamic State, an al-Qaeda offshoot that holds other American hostages and controls territory across northern Iraq and Syria.

After gruesome footage of Foley’s killing appeared on the Internet, Obama warned that the United States would be “relentless” in its response. Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, described the Islamic State as “an apocalyptic, end-of-days” organization that cannot be defeated unless it is “addressed on both sides of what is essentially at this point a nonexistent border.”

But while the border has been exploited by the Islamic State, it has been an impediment to U.S. spy agencies.

U.S. officials cited a failed attempt to rescue Foley and other U.S. hostages in July as indicative of the limits of U.S. intelligence. With information from sources including other hostages who had been released by the group, U.S. spy agencies believed they had identified with high confidence where Foley was being held. But by the time Army commandos arrived at the site, the hostages and their captors were gone.

U.S. officials said spy agencies have similarly singled out compounds where senior Islamic State operatives are believed to have been based. But it has been difficult to sustain surveillance of such targets long enough to be certain that those figures would still be in place if a strike were launched.

Experts said the Pentagon could employ high-flying unmanned aircraft — such as the RQ-4 Global Hawk or stealth drones developed by the CIA — that soar at altitudes beyond the reach of Syrian antiaircraft batteries. But the Global Hawk does not carry missiles or provide the close-in surveillance required to monitor terrorist leaders who move frequently.

At a recent briefing for reporters, U.S. intelligence officials acknowledged that they had scant information even on the whereabouts of Islamic State leader Abu Bakr al-Baghdadi. “We’ve seen him in Mosul,” a U.S. intelligence official quipped, referring to widely circulated Internet footage of Baghdadi’s appearance at a mosque in that Iraqi city to declare himself the leader of a new caliphate. Beyond that, officials said, he is believed to cross frequently between Iraq and Syria.

Some U.S. officials emphasized that even without finding Baghdadi, an air campaign could inflict substantial damage by targeting mid-tier fighters, much the way the CIA drone campaign in Pakistan sought to cripple al-Qaeda by eviscerating its middle ranks. Agency strikes killed a succession of No. 3 operatives who were key to relaying instructions from Osama bin Laden.

In addition to constant drone surveillance in Pakistan, that campaign was also highly dependent on a CIA network of informants that took years to assemble. Current and former U.S. officials said the agency’s effort to train and arm moderate fighters in Syria has had far less impact on affecting the outcome of the civil war than on expanding the agency’s roster of paid assets in the country. “It has morphed into an intelligence collection capability,” a former CIA official said.

A U.S. intelligence official, using one of the acronyms that refer to the Islamic State, said: “We have a good understanding of the strategy, tactics, leadership and organization of the terrorist groups in Syria, including ISIL. . . . As we have demonstrated repeatedly since September 11, 2001, when new threats emerge, or the focus shifts — whether it is in South Asia, Yemen or anywhere else in the world — we can respond to those changes. Nobody should doubt the ingenuity of our officers to collect intelligence, even about difficult places like Syria.”

Others credited the CIA effort but said it remains relatively small in scale. “In Afghanistan, to look across the Pakistan border there were upward of 100 collectors,” meaning U.S. intelligence operatives in remote bases meeting with informants, the senior U.S. intelligence official said. In Syria, “I would say we’re definitely not there.”

Current and former officials said there are limited scenarios in which the Pentagon or CIA could use drones in Syria, including for individual strikes or patrols of remote areas far from the urban settings where Assad’s air defenses are concentrated.

But to maintain long-term surveillance prior to a strike “you are going to have to deal with the air threats” posed by Assad’s military, said retired Lt. Gen. David Deptula, a former head of Air Force intelligence. “That is a much larger set of resources.”

Dempsey said Thursday that the U.S. military was conducting “more than 60” intelligence-gathering flights over Iraq every day. Drones and other aircraft along the Syrian border can peer miles into that country and pick up communications among militant groups.

Sending drones into Syria “could be done and would be a lot less risky than sending pilots over Syrian airspace,” Schiff said. But doing so would probably mean losing some of the remotely piloted aircraft and driving Islamists deeper into areas with dense civilian populations.

“You have to ask what we would accomplish,” he said. “It’s hard to see it providing a major setback to [the Islamic State]. It is easy to see it pulling us into the middle of the horrendous civil war.”

Greg Jaffe contributed to this report.