At a time when North Korea tensions are escalating, the secretary of the Air Force is warning about the capabilities of the branch of the military she oversees.

"The United States Air Force is too small for the missions that the nation is expecting of it. And as a result, we are stretched very thin," Heather Wilson told CNBC in an interview that aired Thursday.

"We've asked to increase the end-strength in the United States Air Force, but we're also putting money into immediate readiness, and then medium and long-term modernization."

Wilson said the Air Force has "turned the corner" in the 2018 budget proposal on "trying to restore" readiness."

The Air Force said it requested a top-line budget of $132.4 billion for investments, including more airmen, readiness, nuclear deterrence operations and cyber capabilities.

Earlier this year, the White House proposed an additional $54 billion in military spending, which would represent a nearly 10 percent increase overall for defense to $603 billion.

— CNBC's Morgan Brennan contributed to this report.