WASHINGTON — The top American air commander in the Middle East is urging the Saudi-led coalition of Arab nations to be more forthcoming about an investigation into an airstrike in northern Yemen earlier this month that struck a school bus, killing more than 40 children.

The comments by Lt. Gen. Jeffrey L. Harrigian reflected increasing exasperation by United States officials over the conflict that has spiraled into a humanitarian disaster.

“There’s a level of frustration we need to acknowledge,” General Harrigian said in a wide-ranging telephone interview last week. “They need to come out and say what occurred there.”

His comments were the latest, and bluntest, public critique of the Saudi-led air campaign against Houthi rebels that have been voiced by a growing number of senior American military, Pentagon and diplomatic officials seeking to distance the United States from the conflict — but who are facing increasing questions about American support for the air war with every errant strike.