The comments from Deborah Lee James come just days after Pentagon chief Ashton Carter raised the possibility of deploying additional US special forces personnel to Syria if more partners can be found among local forces on the ground.
"Air power is extremely important. It can do a lot but it can't do everything," James said.
"Ultimately it cannot occupy territory and very importantly it cannot govern territory," she told reporters at the Dubai Airshow.
James cited the "Iraqi army, the Free Syrians and the Kurds" as forces to support in the fight against IS.
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A US-led coalition has been carrying out air strikes against IS in Iraq and Syria for more than a year in a campaign that has seen the jihadists lose some territory but also make new gains.
The White House announced on October 30 that US President Barack Obama had authorised the first sustained deployment of "fewer than 50" special force personnel to Syria, reversing a longstanding refusal to put US boots on the ground.
While US troops are believed to have carried out covert missions in Syria before, they had not previously been deployed there on a continuous basis.