But with few spies in the city, American officials say assessing the enemy is difficult.
“We’ve spent a lot of time trying to understand the situation on the ground in Raqqa,” Lt. Gen. Jeffrey L. Harrigian, the air war commander, said in an interview from his headquarters in Qatar. “It’s improving. It’s still not at the level we’d like it to be.”
The air operation is a pivotal component of a military campaign that has cost $12.5 million a day in Iraq and Syria. The effort has destroyed hundreds of tanks, artillery pieces, military vehicles, command centers and fighting positions, and killed more than 50,000 fighters, according to American estimates. Since the air war began in late summer 2014, American and allied aircraft have conducted about 17,000 strikes in both countries.
The Islamic State has lost about half of the territory it seized in Iraq and Syria in 2014. But as ISIS loses ground in its physical caliphate, or religious state, the threat of hundreds of foreign fighters returning home and of the expansion of its virtual caliphate through social media is certain to accelerate, American and European officials say. That raises fears of more terrorist attacks in cities outside the Middle East.
For instance, the Islamic State has claimed responsibility for last week’s truck attack on a Christmas market in Berlin even though the links between the group and the main suspect, Anis Amri, a 24-year-old Tunisian, are not completely clear. After Mr. Amri’s death, the Islamic State released a video of him pledging allegiance to the group.
President Obama has vowed to deal the Islamic State crippling blows in Mosul and Raqqa before he leaves office. This month, he ordered 200 more American Special Operations forces to Syria to help local fighters advancing on Raqqa, nearly doubling the Pentagon’s boots on the ground there. Commanders are uncertain, however, about the level of support President-elect Donald J. Trump will maintain for rebel groups in Syria combating the Islamic State.
The military march on Raqqa is complicated by the predominant role played by Kurdish militia members, who make up a majority of the 45,000 fighters bearing down on the city. They are the most effective American partner against the Islamic State in Syria, providing logistics, command and control, and fierce fighting prowess. But the Kurdish fighters are viewed by Turkey — a pivotal American ally — as a terrorist threat.
These lingering diplomatic and military questions leave some congressional leaders voicing skepticism about a swift, decisive attack on the Islamic State capital. “It’s hard to see anything is imminent,” said Representative Adam Schiff of California, the senior Democrat on the House Intelligence Committee.
With a prewar population of about 220,000, Raqqa is about one-tenth the size of Mosul, but commanders still face the same challenges of waging an air war while minimizing risks to civilians in a congested city.
There are other reasons to go slow. Some Islamic State headquarters buildings have been spared attack for now so the Americans can monitor their communications and movements of their personnel in and out to learn more about the enemy operations, General Harrigian said.
Still, allied airstrikes have picked up as the Arab and Kurdish fighters have moved closer to the capital, and as commanders seek to pressure Mosul and Raqqa simultaneously. About 30 percent of the 1,300 strikes in and around Raqqa since the war began in 2014 have been conducted in the past three months.
“The pressure in Raqqa is bearing fruit as ISIL leaders come out of hiding, which allows us to kill them,” Brett H. McGurk, Mr. Obama’s envoy to the international coalition fighting the Islamic State, said this month.
Tracking the enemy’s ground movements falls largely to the crew of the Joint Stars plane, a 1960s-era, reconfigured Boeing 707 jetliner packed with sensitive electronics that is part of an eclectic and unsung mix of odd-shaped surveillance and reconnaissance aircraft with names like Compass Call and Rivet Joint. These planes suck up some enemy communications, jam others and help paint a picture of the Islamic State on the ground for American fighters and bombers to attack.
Bulging from the belly of the Joint Stars is a canoe-shaped, cloud-piercing radar that can see ground targets — and even some low-flying planes and helicopters — as far as 250 miles away on either side of the nearly windowless fuselage.
Aboard the plane, the crew of 19 Air Force and Army personnel — an unusual mix of active-duty and Georgia National Guard specialists — track clusters of dots on their screens that could represent groups of hostile fighters and their vehicles, friendly forces or just routine commercial traffic. Much depends on where they are and what time of day it is.
From its high-flying, wide-area perch, the radar can track moving vehicles; low, slow-flying aircraft; and smaller potential targets such as people, said Lt. Col. William B. Hartman, 39, of Irvine, Calif., the Joint Stars squadron commander. The operators on board can change the filters on their systems to show different-size targets or their direction of travel in different colors, all of which is relayed back to operations centers in Baghdad and Erbil, in northern Iraq, he said.
Flying from a base in the Persian Gulf, a typical Joint Stars mission over Iraq, Syria or Afghanistan can last about 11 or 12 hours. Crews pack snacks in their flight bags but also fire up the plane’s oven to prepare an in-flight order of chicken wings.
The Joint Stars is not equipped with cameras to identify specific images on the ground. When the crew members see something suspicious, they direct a Predator or Reaper drone to zoom in for a closer look. The Joint Stars is also valuable because a rotation of aircraft and surveillance crews can monitor a particular area for days, weeks or months, watching Islamic State activity to understand what the military calls the enemy’s “pattern of life.”
Islamic State fighters know from experience that they are being watched and often try to deceive the surveillance planes, hiding in schools or mosques or using camouflage. At one point, analysts said, ISIS even appeared to be trying to smuggle weapons strapped to the bellies of herds of sheep.
“They’re extremely smart,” said Master Sgt. Caylon Kimball, 31, an airborne intelligence technician from Anadarko, Okla.
Several weeks ago, as the air campaign intensified against the Islamic State’s oil-production and distribution network, analysts noticed an intriguing development in the central Syrian desert, about 35 miles north of Palmyra.
Comparing months-old radar data from Joint Stars and other surveillance imagery with newer versions, analysts discovered that the Islamic State was moving much of its oil tanker truck fleet to an obscure area of sandy gullies, about 20 miles by 20 miles in size.
“They were trying to hide from us,” General Harrigian said. “They were adapting to what we were doing. They were going into the desert and just parking.”
For several more weeks, analysts watched the clandestine desert truck stop grow, wanting to ensure it was the Islamic State trucking fleet. Confident in that assessment, General Harrigian ordered an attack plan, code-named Olympus. In two waves of strikes — on Dec. 8 and 9 — about two dozen Air Force and Navy warplanes destroyed 188 of the trucks. Empty truck cabs were struck first to scare off drivers sleeping in their rigs, and General Harrigian said it appeared there were no civilian casualties.
Besides wiping out a sizable portion of the Islamic State’s tanker truck fleet and depriving the group of over $2 million in oil sales, commanders said the strike was also meant to cripple morale.
“There would be a larger strategic message we sent to them: Nice try. We found you,” General Harrigian said. “Keep trying to hide, we will hunt you down again.”