The air campaign against the Islamic State (ISIS) terrorist group has made progress in degrading the jihadist organisation but will not be enough without local boots on the ground, the US air force secretary said on Tuesday.
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.
"This is where we need to have boots on the ground. We do need to have ground forces in this campaign."
James cited the "Iraqi army, the Free Syrians and the Kurds" as forces to support in the fight against ISIS.
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 authorized the first sustained deployment of "fewer than 50" special force personnel to Syria, reversing a longstanding refusal to put US boots on the ground.
And in an interview on Sunday, the Pentagon chief said that more US troops could "absolutely" be deployed to Syria if Washington identifies more "capable local forces" as partners in the fight against ISIS.
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.
AFP contributed to this report.