A U.S. B-1B bomber flies with fighter jets from South Korea and the United States fighter over the Korean Peninsula during last December’s Vigilant Ace air-combat exercise. This year’s has been canceled. (Handout/Getty Images)

The United States and South Korea suspended a high-profile air-power exercise scheduled for December, the fourth such military training operation the two allies have canceled as a result of nuclear disarmament talks with North Korea.

Defense Secretary Jim Mattis and his South Korean counterpart decided to cancel the joint military exercise, known as Vigilant Ace, “to give the diplomatic process every opportunity to continue,” the Pentagon’s chief spokeswoman, Dana W. White, said Friday in a statement.

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

U.S. military exercises with South Korea, which improve capacity and prepare the two allies to work together in the event of a conflict, have long irritated North Korea, which describes them as “war games” and often mounts a negative reaction when they take place. 

In June, President Trump met with North Korean leader Kim Jong Un in Singapore and agreed to suspend some of the exercises as a concession to Pyongyang during the disarmament talks. North Korea has not conducted any intercontinental ballistic missile or nuclear warhead tests since then.

Trump has regularly made statements in recent weeks suggesting that all U.S. military exercises with South Korea had been suspended. But lower-level training operations between the two allies have continued apace.

Only three high-profile exercises had been suspended as a result of the Singapore summit: Ulchi Freedom Guardian and two Korean Marine Exchange Program training operations. Friday’s decision to suspend Vigilant Ace marked the fourth canceled exercise. 

Vigilant Ace is primarily an annual air-combat exercise in which fighter jets from both countries come together and fly in various scenarios. The Pentagon described it as a way to enhance the ability of the armed forces from the two countries to operate with one another and “ensure peace and security on the peninsula.”

Mattis has not been a fan of the decision to suspend the military exercises, according to former U.S. officials, who said that he had urged against putting them on the table during negotiations. The Pentagon sees regular exercises as a critical way to deter would-be enemies and assure allies that Washington will come to their defense in the event of a conflict. 

Trump, however, has long been a skeptic of military exercises and has suggested that the joint training operations the United States carries out with South Korea are a waste of money. The president has also balked at the cost of the nearly 30,000 U.S. troops stationed at bases in South Korea and at one point seemed to threaten to remove them entirely

The Pentagon said Friday that Mattis consulted his counterpart in Japan regarding the suspension of the Vigilant Ace exercise and the two “reaffirmed their commitment to regional security.” 

Though Trump previously has suggested that the United States suspended military exercises with Japan, the Pentagon has said they have been continuing uninterrupted.

This month, Secretary of State Mike Pompeo met with Kim in Pyongyang, and also with leaders in South Korea and China to discuss nuclear disarmament in North Korea. Pompeo has said that he is looking to organize a second summit between Trump and Kim.