The Pentagon is ratcheting up warnings about budget and national-security risks that it contends would result from abruptly ending the use of Russian-built RD-180 engines that power many U.S. military rockets.

In the latest escalation of the long-running debate over the fate of such engines, Deputy Defense Secretary Robert Work told a veteran Democratic lawmaker the Air Force doesn’t have the budget flexibility to cover the cost of rapidly replacing RD-180 engines with more-expensive domestic boosters.

...