AMMAN (Reuters) - A reduction in U.S. troops taking part in a joint exercise with South Korean forces this year simply reflects a need for fewer personnel and is not because of tensions with North Korea, Defense Secretary Jim Mattis said on Sunday.

About 17,500 U.S. service members are participating in the exercise this month, down from 25,000 last year, according to the Pentagon.

With tensions high on the Korean peninsula, China - Pyongyang’s main ally and trading partner - has urged the United States and South Korea to scrap the drills, as has Russia.

Called Ulchi Freedom Guardian, the joint drills will take place from Aug. 21 to Aug. 31 and involve computer simulations designed to prepare for the unthinkable: war with nuclear-capable North Korea.

Mattis told reporters while traveling to Jordan that the exercise had been planned months in advance and the focus this year was on integration operations.

“The numbers are by design to achieve the exercise objectives and you always pick what you want to emphasize,” he said. “Right now there is a heavy emphasis on command post operations, so the integration of all the different efforts,” Mattis said.

North Korea’s rapid progress in developing nuclear weapons and missiles capable of reaching the U.S. mainland has fueled a surge in tension.

President Donald Trump has warned that North Korea will face “fire and fury” if it threatens the United States. The North responded by threatening to fire missiles towards the U.S. Pacific island territory of Guam.

Pyongyang later said it was holding off firing towards Guam while it waited to see what the United States would do next.