The Pentagon announced Friday it is suspending another major military exercise with South Korea in an effort to support denuclearization talks with North Korea, raising concerns as to how long forces on the peninsula can forgo major training opportunities before readiness is hurt.

U.S. Defense Secretary Jim Mattis and South Korean National Defense Minister Jeong Kyeong-doo made the decision to suspend “Vigilant Ace,” an annual December air exercise involving more than 12,000 forces, “to give the diplomatic process every opportunity to continue,” Pentagon press secretary Dana White said in a statement.

“Both ministers are committed to modifying training exercises to ensure the readiness of our forces. They pledged to maintain close coordination and evaluate future exercises,” she said.

It’s been almost a year since North Korea tested its last ballistic missile, after firing missiles throughout most of 2017 that proved the country had rapidly advanced its missile capabilities.

In the months of quiet since then, President Donald Trump has made repeated overtures to the North Korean government, which resulted in a summit between the two countries last summer in Singapore. At that summit, Trump pledged to suspend what he perceived as wasteful wargames that North Korea views as provocative; North Korea in the months since has re-started the repatriation of U.S. remains from the Korean War.

Mattis is in Singapore meeting with Asian ministers of defense including his Japanese counterpart Takeshi Iwaya. In the statement, White said Mattis consulted with the Japanese on the decision.

The announcement is the latest twist around the question of when, or if, the U.S. will resume the joint exercises, and with warnings that the pause in exercises will hurt the readiness of U.S. forces on the peninsula.

During a Sept. 25 hearing, Army Gen. Robert Abrams, the then-nominee to lead U.S. Forces-Korea, said of the Ulchi Freedom exercise being canceled: “That’s a key exercise to maintain continuity and to continue to practice our interoperability, and so there was a slight degradation” in readiness.

A day later, Mattis downplayed that impact, saying: “If you emphasize the word ‘slight,’ certainly if you’re not training today then you could say there’s a slight degradation. Is it notable? Is it material? ... I think that’s why he [Abrams] put the word ‘slight’ in there.

“There’s nothing significant to it.”

But it’s the compounding effect of multiple cancellations — and Friday’s announcement marked the fourth canceled or suspended exercise — that will hurt U.S. forces, said retired Army Lt. Gen. Tom Spoehr, who commanded U.S. Forces-Iraq and is now a fellow at The Heritage Foundation.

“There’s already been a readiness degradation, is the truth of the matter,” Spoehr said. At this point the degradation is still probably acceptable, he said. However, the nature of the U.S. rotations in Korea — in one-year unaccompanied or two-year accompanied posts, depending on rank — means the relationships and ability to understand each other’s decision-making and operations is perishable.

“That readiness, that ability to do that [be fully interoperable] is already starting to age,” he said.

Last year’s Vigilant Ace involved 230 fighter and support aircraft, including F-22 Raptors, F-35 Lightning IIs, F-16 Fighting Falcons, F-15 Eagles, F-18 Hornets and EA-18G Growlers “flying alongside ROK F-15K Slam Eagles and F-4 Phantom IIs, providing realistic air combat training and enhancing operational and tactical-level coordination through combined and joint combat training,” according to a DoD release on the December 2017 exercises. ROK stands for the Republic of Korea - another term for South Korea.

The Pentagon said that it would look for modified training opportunities to keep readiness current. That could mean that instead of a massive air wing-sized gathering of aircraft, regular missions of between squadron-sized Korean and U.S. aircraft could be stepped up, Spoehr said.

Earlier this year, the Pentagon suspended Ulchi Freeedom Guardian, an annual August-September exercise involves 17,500 U.S. forces and last year included almost two weeks of exercises with their South Korean counterparts. However, the status of two major exercises traditionally scheduled for the spring — Foal Eagle and Max Thunder, major ground and joint air exercises that help improve South Korean and U.S. interoperability — has been unclear.

On Aug. 28, Mattis told reporters there were “no plans, at this time, to suspend any more exercises” on the Korean Peninsula, adding that the department has “done no planning for suspending” future exercises.

Asked specifically if the next Foal Guardian would go forward, Mattis reiterated that “we have not made decisions on that at this time, and we will do that with consultation with [the] State [Department].”

That was seen as a sign that the Pentagon intended to resume the regular exercises come 2019. But one day later, Trump took to Twitter to make it clear exercises were still off the table.

In a “Statement from the White House” that Trump tweeted from his personal account, but then referred to himself in the third person, the president said: “the President believes that his relationship with Kim Jong Un is a very good and warm one, and there is no reason at this time to be spending large amounts of money on joint U.S.-South Korea war games."