Secretary of Defense Ash Carter on Tuesday penned a letter to Congress urging them to pass a full appropriations bill rather than confine the Defense Department to fiscal 2016 spending levels until May of next year.

“I urge Congress to keep the CR (continuing resolution) as short as possible and finish full-year appropriations,” Carter wrote in the letter, sent Nov. 29 to Senate Majority Leader Mitch McConnell (R-Ky.) and House Speaker Paul Ryan (R-Wis.).

Carter has repeatedly and publicly urged Congress to buck the trend of funding the government in several-month increments instead of providing budget predictability that comes with a full appropriation. The Defense Department has begun the last eight fiscal years under a CR and could make history by undergoing a presidential transition under a stopgap funding measure.

Carter said he is “particularly troubled” by the possibility Congress will enact a CR that lasts another six months.

“A short-term CR is bad enough, but a CR through May means DoD would have to operate under its constraints for two-thirds of the fiscal year,” Carter wrote. “This is unprecedented and unacceptable, especially when we have so many troops operating in harm’s way. I strongly urge Congress to reject this approach.”

Pentagon spokesman Peter Cook underscored Carter’s concern during a press conference on Tuesday. A CR lasting until May could cut short funds needed for the ongoing fight against the Islamic State, also called ISIL, in Iraq and Syria, he said.

“One task that an extended CR would make more difficult is the fight against ISIL, which would be deeply unfortunate because we continue to achieve results in that fight,” Peter Cook said. Training and readiness for stateside troops preparing for deployment also would be put at risk, he added.

Funding the Defense Department with a series of CRs instead of long-term budget plans is inefficient, impractical and “certainly not and effective way to plan for the future,” Cook added.

“We continue to work closely with Congress,” Cook said. “We know there is bipartisan support for our operations overseas, particularly the fight against ISIL and our operations in Afghanistan and so we remain optimistic that we will be able to work this out with members of Congress. “

Shortfalls will be most prominent in accounts that pay for ongoing operations and munitions used in those fights in Afghanistan, Iraq and Syria, Carter wrote. The European Reassurance Initiative – which is set to quadruple in fiscal 2017 – under a CR will be capped at fiscal 2016 funding levels.

“These are areas we cannot afford to shortchange,” he said.

Under a CR, the Defense Department has no authority to launch new programs, boost production rates or begin multi-year procurement efforts. The Pentagon has at least 57 new-start programs and 86 production increases to existing programs planned for fiscal 2017, Carter wrote.

“As a result the CR would undermine critical programs such as the KC-46 tanker, Apache and Black Hawk helicopter procurements and the Ohio replacement submarine,” Carter wrote. “Failure to continue these programs as planned will cost the taxpayer hundreds of millions of dollars in needless contractual penalties.”