Acting Secretary of Defence Christopher Miller made the surprising comments to press as he returned from a trip to Nebraska, Tennessee and Colorado.
Mr Miller, who has only been in the job for two months, spoke out when he was asked about the F-35 Joint Strike Fighter, the most expensive weapons system in history.
"I so... I mean, I cannot wait to leave this job, believe me,” he replied according to a Department of Defence transcript.
Mr Miller then appeared to disparage the $1 trillion fighter jet programme.
"Yesterday we were talking to some guy, some lieutenant colonel, or colonel, said 'what are you flying?' Said 'F-35,' I was like that's a piece of ... and he was like ... and he laughed, and I was like, ‘No seriously, tell me about it,’ he told reporters.
“I'm like, we have created a monster, but you know that.”
Mr Miller then went on to praise the success Russia has had during the Trump administration.
"I kind of, you know, like professionally I'm like, wow, they're doing pretty well, and they're using a lot of irregular warfare concepts, information, all this stuff, in a way that, you know, like ... good on them," he said.
A Pentagon official travelling with Mr Miller had to clarify his comments.
“The Secretary often uses casual and humorous language with reporters and personnel during travel. That characteristic does not convey well in a written transcript but was obvious to participants,” said the official.
Mr Miller got the job when the outgoing president fired Mark Esper on Twitter on 9 November.