Joseph Votel, the top American general in the Middle East, also said that the US-backed forces on the ground in Syria were not ready to handle the threat of ISIS on their own.
"It would not have been my military advice at that particular time ... I would not have made that suggestion, frankly," Votel said of the troop withdrawal. "(The caliphate) still has leaders, still has fighters, it still has facilitators, it still has resources, so our continued military pressure is necessary to continue to go after that network."
"When I say, 'we have defeated them,' I want to ensure that means they do not have the capability to plot or direct attacks against the US or our allies," Votel said. "They still have this very powerful ideology, so they can inspire."
The commander of US Central Command had previously said that he "was not consulted" before Trump's controversial announcement late last year that the US would rapidly withdraw its troops from the war-torn country.
Trump's announcement appalled US lawmakers and triggered resignations, including that of Defense Secretary James Mattis and the senior State Department official in charge of the anti-ISIS campaign.
"They still require our enablement and our assistance with this," Votel said, adding that the US military was still in the midst of executing a "well-crafted military campaign."
"We want (ISIS) to be able to be controlled or addressed by the indigenous partners, whether that's the Iraqi security forces in Iraq, or the Syrian Democratic Forces in Syria, that when they are capable of handing this threat on their own, without our assistance, that will be another key criteria indicating to me that we have accomplished our mission of defeat of ISIS."
Votel also said Iran posed a growing threat to US partners in the region and expressed concern over its development of advanced weapons, though he acknowledged that Tehran had held up its side of the nuclear deal that Trump withdrew from last year.
"I acknowledged that the reporting is that they remain in compliance with the provisions of the JCPOA [the nuclear deal], but of course we do see them continuing to advance their missile technology," Votel said. "It should not be lost on anybody that an advanced ballistic missile program could also be used to move weapons of mass destruction."
"Their ability to be innovative, their ability to seek more precision, and the fact that they are increasing their quantity are the greatest areas of concern for me," he added.
"They are looking to use new ways of orchestrating their actions so it isn't so much so through just ballistic missiles, but we've also seen a proliferation of unmanned aerial systems ... that operate in different (ways), so this could challenge us."
Votel's comments come a day after Vice President Mike Pence called on a gathering of 62 countries in Poland to confront Iran directly, and ripped into Europe for its refusal to pull out of the nuclear deal.