NELLIS AIR FORCE BASE, Nev. – The Air Force’s top civilian leader said she “flat-out” believes there is no trust by the United States in Russia’s operations in Syria, becoming the latest senior U.S. defense official to raise concerns about a proposal under which U.S. and Russian forces would collaborate to carry out some airstrikes in Syria.

Air Force Secretary Deborah James said that Russia’s lack of truthfulness gives her pause in assessing how a proposal under discussion would work. She cited Russia’s claims that it is in Syria to strike the Islamic State despite focusing on other armed groups that are opposed to Syrian President Bashar al-Assad, and Moscow’s use of “dumb bombs” in Syria that are difficult to control and elevate civilian casualties.

“When you have a say-do gap as we do with the Russians, there is just no trust there, is what I would flat-out say,” James said in an interview Thursday on this air base. “Now, of course, if we get this agreement, it won’t be based on trust. It will be based on transaction. But nonetheless, I think it will present some difficulties.”

James’s comments are the most pointed yet by a senior U.S. defense official on the proposed plan, in which the United States and Russia would coordinate how to strike the former Jabhat al-Nusra, which has been linked to the al-Qaeda affiliate in Syria, and synchronize their operations against the Islamic State. To date, U.S. and Russian forces communicate to “deconflict” aviation operations over Syria to avoid collisions and other kinds of mishaps, but do not share information about future operations.

Jabhat al-Nusra split from al-Qaeda on Thursday, taking the new name Jabhat Fatah al-Sham. The decision was widely interpreted to be an effort to head off any joint Russian-U.S. airstrikes.

The Obama administration, particularly Secretary of State John F. Kerry, has offered closer collaboration with the Russians in exchange for Moscow pressuring the Assad regime to halt its airstrikes against Syrian people and limit its ground operations to humanitarian and medical missions. But the proposal has drawn a wide range of concerns and skepticism from senior U.S. defense officials.

Defense Secretary Ashton B. Carter and Marine Gen. Joseph F. Dunford, the chairman of the Joint Chiefs of Staff, expressed reservations during a July 25 news conference. Both men raised a lack of trust as an issue, and Dunford added that there will be “specific procedures and processes in any transaction we might have with the Russians that would account for protecting our operational security.”

But James, seated alongside Gen. David L. Goldfein, the new chief of staff of the Air Force, went even further. Asked if she had concerns about how it would look for Russian and U.S. forces to be collaborating, she said she doesn’t “worry about the optics,” but noted that Russia would be unlike other partners the United States has in the war against the Islamic State.

James and Goldfein said that if the administration decides that greater collaboration is a good idea, they are confident that U.S. combat pilots will out the jobs they are assigned. Goldfein added that it could potentially boost the safety of pilots if the Russians are committed to greater communication.