The comments by Gen. Qassem Suleimani, the head of the Guard's elite Quds unit, come just after US Defense Secretary Ash Carter accused Iraqi forces of lacking the "will to fight" in an interview aired the day before.
It wasn't clear whether Suleimani's remarks came as a direct response to Carter's, though tensions remain high between the two countries amid negotiations over Iran's contested nuclear program.
The report in the daily newspaper Javan, which is seen as close to the Guard, quoted Suleimani as saying the US didn't do a "damn thing" to stop the extremists' advance on Ramadi.
"Does it mean anything else than being an accomplice in the plot?" he reportedly asked, later saying the US showed "no will" in fighting the Islamic State group.
Suleimani said Iran and its allies are the only forces that can deal with ISIS,
"Today, there is nobody in confrontation with ISIS except the Islamic Republic of Iran, as well as nations who are next to Iran or supported by Iran," he said.
US officials, including Carter, have said Iraqi forces fled the Islamic State advance on Ramadi without fighting back, leaving behind weapons and vehicles for the extremists. So far, the American approach to the conflict has been to launch airstrikes as part of an international coalition it leads, as well as equipping and training Iraqi forces.
Iran has offered advisers, including Suleimani, to direct Shiite militias fighting against the extremists. Iran has said it does not have combat troops fighting in Iraq, though some Revolutionary Guard members have been killed there.
Meanwhile in Syria, activists said government warplanes conducted more than 15 air raids on the historic central town of Palmyra and nearby areas, leaving some dead or wounded.
The air raids on Palmyra came a day after the government said that Islamic State fighters have killed more than 400 state employees, troops and pro-government gunmen since they captured the town Wednesday.
The Britain-based Syrian Observatory for Human Rights and Osama al-Khatib, an activist from Palmyra who is now in Turkey, said the warplanes also struck near a building where archaeological pieces were placed before they were moved to safe areas.
The activists said the air raids were mostly inside the town and far from the famous archaeological sites that are among the Middle East's most spectacular.