WASHINGTON — Defense Secretary Ash Carter has ordered Navy leaders to buy fewer ships so the service can spend more on jets such as the F-35 as well as munitions and upgraded systems for electronic warfare.

“For the last several years, the Department of the Navy has overemphasized resources used to incrementally increase total ship numbers at the expense of critically needed investments in areas where our adversaries are not standing still, such as strike, ship survivability, electronic warfare and other capabilities,” Carter wrote in a memo obtained by Bloomberg News that’s rare in its blunt rejection of a military service’s approach.

Carter said the Navy is well on its way to reaching a 308-ship goal that “should be met but not irresponsibly exceeded.” The size of the naval fleet has long been a political flashpoint. In the 2012 presidential race, Republican Mitt Romney criticized President Barack Obama for letting the number of ships shrink to “levels not seen since 1916.”

Navy spokesman Cmdr. William Marks said service officials were aware of the memo but “budget discussion are pre-decisional and it would inappropriate to discuss anything further until the FY 17 budget is finalized.”

“Shipbuilding has always been a priority for the Navy,” Marks said. “We will continue to balance capability with capacity in our shipbuilding programs as we have always done.”

In the memo to Navy Secretary Ray Mabus, Carter said the Navy’s upcoming five-year plan for fiscal 2017 to 2021 should reduce the combined purchases of Littoral Combat Ships and a new frigate to 40 from 52. The littoral ship, made in separate versions by Lockheed Martin and Austal, has been dogged by questions about its mission, performance and ability to survive in combat.

At the same time, Carter directed the Navy to increase purchases over the five years of Lockheed’s F-35 and the F/A-18E/F made by Boeing. Thirty-one of the Navy’s version of the F-35 would be added over the Navy’s request.

He also called for more Raytheon Co. SM-6 missiles, a new lightweight torpedo, upgraded electronic warfare systems on surface ships and increased compartment space to carry more Tomahawk cruise missiles on Virginia-class submarines. Carter praised the system, known as the “Virginia Payload Module.”

“The Navy’s strategic future requires more on focusing on posture, not only on presence, and more on new capabilities, not only on new ship numbers,” Carter said.

With the Obama administration ending in little more than a year and Carter’s tenure at the Pentagon likely to end then as well, the Navy and its supporters in Congress may simply outlast his directive.

Questions about the Littoral Combat Ship’s survivability and mission led former Defense Secretary Chuck Hagel a year ago to truncate the number of ships planned at 32, to be followed after 2019 by development and purchase of 20 better-armed frigates But the Littoral’s political survivability is buoyed by strong support on Capitol Hill.

Carter’s memo directed the Navy to cut the number of Littoral Combat Ships in its plan for fiscal 2017 to 2021 to six from 14. He also directed the Navy to choose between Lockheed and Austal by fiscal 2019 to build future ships, rather than continuing to turn out two variations.