WASHINGTON — Warplanes from the United States, Saudi Arabia and the United Arab Emirates on Thursday attacked what military officials believe were the majority of the Islamic State’s oil refineries in Syria as part of the continuing effort to target sources of the terrorist group’s financing, Pentagon officials said.

The strikes on the 12 small refineries came on the third day of the American-led air campaign in Syria, and early reports indicated that the attacks had crippled the plants in the eastern provinces of Deir al-Zour and Hasakah, Rear Adm. John Kirby, the Pentagon press secretary, said at a news conference.

He played a video and displayed aerial photographs of the strikes, which in at least one case left a refinery tower standing while destroying the buildings around it.

Still, Admiral Kirby and other military officials acknowledged shortcomings in the budding effort to roll back gains by the Islamic State, particularly given that there were no plans to send in American troops to capitalize on airstrikes.