Defense Secretary Ash Carter said Wednesday that he's deeply troubled by a House Republican proposal to reduce wartime funding by $18 billion to pay for weapons and troops the Pentagon didn't request.

Testifying before the Senate Appropriations defense subcommittee, Carter said the plan advocated by Rep. Mac Thornberry of Texas risks money that is critical for warfighting missions at a time when U.S. troops are deployed in fights against the Islamic State group and other extremists.

"While I don't expect this committee to consider such a proposal, I have to say that this approach is deeply troubling, and flawed for several reasons," Carter said. "It's gambling with warfighting money at a time of war -- proposing to cut off our troops' funding in places like Afghanistan, Iraq, and Syria in the middle of the year."

Thornberry, the chairman of the House Armed Services Committee, said his proposal would fill gaping shortfalls in the Obama administration's $583 billion defense budget that forces U.S. military commanders to cut training hours, defer the maintenance of equipment and forestall the purchase of new weapons.

"The bottom line for me this year is that it is fundamentally wrong to send service members out on missions for which they are not fully prepared or fully supported," Thornberry said in his opening statement as the House Armed Services Committee met Wednesday to consider the annual defense policy bill.

The bill, which includes his plan to shift the wartime money, authorizes defense spending for the fiscal year that begins Oct. 1.

Thornberry's plan would pay for additional ships, jet fighters, helicopters and more. It also prohibits the Army from falling below 480,000 active-duty soldiers and adds 7,000 service members to the Air Force and Marine Corps.

But there's a hitch in Thornberry's plan: He needs help from President Barack Obama's successor. To make up for the war spending shortfall, the new president will have to submit a supplemental budget to Congress in early 2017 before the wartime account runs out.

"Starting to turn around our readiness shortfalls while staying within the total dollars requested by the administration means that there is not enough money to fund the (wartime) activities proposed by the president for the full year," Thornberry said. "There will be enough for roughly six months."

Carter told the Senate subcommittee that Thornberry's plan actually degrades combat readiness by retaining troops and buying equipment that can't be sustained.

"It's another road to nowhere, with uncertain chances of ever becoming law, and a high probability of leading to more gridlock and another continuing resolution ... exactly the kind of terrible distraction we've seen for years, that undercuts stable planning and efficient use of taxpayer dollars, dispirits troops and their families, baffles friends, and emboldens foes," Carter said. "I cannot support such maneuvers as secretary of defense."