President Trump said on Sunday that he had instructed the Pentagon to reduce joint military exercises with South Korea, a longtime U.S. ally and a linchpin of America’s national security strategy in the Asia-Pacific region.

The announcement may heighten growing worries among Asian allies over what the United States is now willing to do to deter China, given the enormous distraction and resource drain of the U.S.-Israel war against Iran. The United States is running low on critical weapons and has been moving military resources out of Asia to the Middle East.

In a convoluted social media post, Mr. Trump complained that Seoul did not “join us in the Denuclearization of the Islamic Republic of Iran.” In the same post, he said that North Korea under Kim Jong Un had been “unthreatening and respectful” while he has been president.

In his post Mr. Trump did not mention a rift in relations in 2017, when the North Korean leader called him a “mentally deranged dotard” after Mr. Trump delivered a speech at the U.N. threatening to “totally destroy” North Korea if the United States had to defend itself or its allies.

Now Mr. Trump is speaking only of what he on Sunday called his “very good relationship” with North Korea and its leader. In his post, he said that because of those ties, he was “not happy with the fact that the United States has, long ago, agreed to participate in Joint Military exercises with South Korea.”

“These exercises are not only costly, with much of these costs paid for by the United States of America (as usual!), but send a signal that is totally inappropriate and hostile, to a Country that, as long as Donald J. Trump has been President, has been unthreatening and respectful,” Mr. Trump said in his post.

Mr. Trump said that because it was too late to cancel the exercises entirely, he had asked Defense Secretary Pete Hegseth to “substantially reduce” U.S. participation.

The U.S. and South Korean militaries have held joint exercises for more than 70 years, since 1955, after the 1953 armistice that ended the Korean War.

It was unclear just how the Pentagon would scale back its role in the joint military exercises, called Ulchi Freedom Shield, which are set to begin on Monday and run for 11 days.

The exercises are held most summers with South Korea’s armed forces and with other U.S. allies in the Asia-Pacific region. North Korea has always decried the exercises as dress rehearsals for an invasion.

The exercises focus on how the allied militaries can counter a wide range of security threats, including drones and cyberattacks, while also strengthening combined defenses. They are designed to help U.S. allies defend against threats from North Korea.

The Pentagon was taken aback by Mr. Trump’s sudden announcement, according to a senior U.S. military official. Just last week, an official with U.S. and South Korean Combined Forces Command told reporters that the exercises would simulate combat against North Korean troops, who have gained crucial experience while fighting for Russia in its war against Ukraine.

The exercises are supposed to include 18,000 South Korean troops, as well as a number of U.S. troops who are already part of the 28,500 American service members stationed in South Korea.