The Pentagon said late on Tuesday that it was ending a major training exercise with South Korea one week earlier than scheduled, on Aug. 21, and scaling back many remaining live-fire training drills to computer simulations or canceling them altogether.

The moves came after President Trump on Sunday ordered the Defense Department 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.

Mr. Trump’s order was aimed at rekindling talks with Kim Jong-un of North Korea at a time when efforts to resolve the Iran war have hit an impasse, as well as punishing the South Koreans for declining to join in the conflict against Iran, a position that deeply angered Mr. Trump.

In a statement released on Tuesday night, the Pentagon said it was sharply curtailing the 11-day training exercise in South Korea, called Ulchi Freedom Shield, “while continuing training that enhances tactical capability.”

“These adjustments preserve essential readiness and training objectives,” the statement continued. “There will be no degradation to U.S. training objectives.”

Even before the Pentagon’s announcement, Mr. Trump’s order had heightened fears among Asian allies over what the United States is now willing to do to deter China in its territorial ambitions, given the enormous distraction and resource drain of the Iran war. The United States is running low on crucial weapons and has been moving military resources from Asia to the Middle East.

Current and former U.S. military commanders have said that abruptly scaling back the duration of the exercise and converting many of its live-fire components to computer simulations would not only stoke those fears but also erode American military readiness on the highly militarized Korean Peninsula.

In Seoul on Wednesday, ⁠South Korea’s foreign minister, Cho Hyun, said at a parliamentary ⁠hearing that neither his country nor the United States had been given advance warning about Mr. Trump’s order on Sunday to scale back joint military drills.

“Neither our government nor U.S. officials had any prior knowledge of what President Trump revealed,” Mr. Cho said. “We received no advance notice.”

The exercises are held most summers with South Korea’s armed forces and with other U.S. allies in the Asia-Pacific region. They 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.

Col. Ryan Donald, director of public affairs for the Combined Forces Command, United Nations Command and U.S. Forces Korea, said in a news conference with the South Korean on Aug. 10 that the exercises reflected the changing character of war, including the growing use of unmanned systems, electronic warfare and cyberoperations.

In the convoluted social media post, Mr. Trump complained on Sunday 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 Mr. Kim had been “unthreatening and respectful” while he has been president.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.