ABOARD A JOINT STARS SURVEILLANCE PLANE, Over Northern Iraq — Flying at 30,000 feet, the powerful radar aboard this Air Force jet peered deep into Syrian territory, hunting for targets on the ground to strike in the looming offensive to seize Raqqa, the Islamic State’s capital.

It was on a mission like this several weeks ago that analysts discovered a hiding place in the central Syrian desert where the Islamic State was stashing scores of oil tanker trucks that provide the terrorist group with a crucial financial lifeline. Acting on that tip and other intelligence, two dozen American warplanes destroyed 188 of the trucks in the biggest airstrike of the year, eliminating an estimated $2 million in oil revenue for the Islamic State, also known as ISIS or ISIL.

Even as the American-led air campaign conducts bombing missions to support Iraqi troops fighting the Islamic State in Mosul, American commanders said the air war would probably play an even greater role in Syria over the coming weeks in the battle to retake Raqqa.

Newly recruited Syrian Arab militia fighters, allied with experienced Kurdish fighters, are encircling Raqqa. But they need allied bombing to weaken and dislodge enemy forces dug in there, and to cut off the ability for the Islamic State to rearm, refuel and reinforce its fighters.