U.S. Air Force warns F-35U.S. Air Force Chief of Staff General Mark Welsh on Tuesday warned that short-term moves to revise downward the Air Force’s planned purchase of 1,763 Lockheed Martin Corp F-35 stealth fighters could damage the program and scare off foreign buyers.

“Let’s delay this discussion for a little while until we see what happens in the world,” Welsh told reporters at the annual Air Force Association conference, citing efforts by Russia and China to field more advanced fighter aircraft of their own.

“All we do right now is risk damaging a program that’s now gaining some momentum and is about to become operational,” he said.

Marine Corps General Joseph Dunford, who takes over as chairman of the Joint Chiefs of Staff this month, sparked questions about possible changes to the U.S. military’s plans for a total fleet of 2,457 F-35s during his U.S. Senate confirmation hearing. The Pentagon later said no formal review was underway.

Admiral John Richardson, who takes over as chief of naval operations on Friday, also told lawmakers he would take a hard look at the Navy’s current requirement for 340 F-35 C-model jets that can take off and land on aircraft carriers.

The F-35 is the Pentagon’s largest arms program, with an estimated price tag for development and production of $391 billion over the next five decades.

Lockheed is developing and building three models of the F-35 for the U.S. military and nine countries that have placed orders: Britain, Australia, Italy, Turkey, Norway, the Netherlands, Israel, Japan and South Korea. Canada and Denmark, which helped fund the jet’s development, are expected to decide about possible F-35 purchases in coming months.

The Pentagon’s F-35 program manager, Air Force Lieutenant General Chris Bogdan, told a separate panel he was determined to keep the jet on track for the Air Force to declare an initial squadron of F-35s ready for combat by Aug. 1, 2016.

Welsh told reporters the program was doing well after years of cost overruns and technical challenges, and it was critical to keep moving toward full production to ensure continued cost reductions. He said the jet was likely to cost about $80 million per plane by 2019, comparable to other fighter jets.

Bogdan told Reuters that none of the services had asked the his office to participate in a reassessment of their overall F-35 procurement plans, and he did not expect them to carry out revisions in the near future.