The Pentagon’s top spokesman Wednesday suggested that Syria’s sophisticated air defense system didn't pick up U.S. forces entering its air space during a failed mission to rescue journalist James Foley and other prisoners held by Islamic extremists.

Rear Adm. John Kirby, the Pentagon's press secretary, was discussing the military’s decision-making process when it comes to conducting aerial surveillance over Iraq and Syria with NBC’s Andrea Mitchell.

Kirby was explaining the importance of surveillance flights in monitoring the threat a group like the Islamic State of Iraq and Syria poses.

Mitchell then pointed out that Syria’s “robust” Russian-financed air defense system didn’t detect and start targeting U.S. forces when they crossed into the country to try to rescue Foley and the other ISIS prisoners early this summer.

“Right,” Kirby said, seeming to confirm that account.

“What is the speculation as to why the Assad regime did not stop those plans, choppers?” Mitchell asked.

Kirby responded only that it was a “very planned mission, very quietly planned, for obvious reasons” and executed in a “very stealthy way.”

“I wouldn’t get into the details beyond that,” he concluded.

U.S. special forces secretly entered Syria earlier this summer in a risky and ultimately unsuccessful mission to rescue Foley and other ISIS prisoners. Last week, ISIS forces circulated a video showing Foley’s beheading.