US President Donald Trump said he ordered the Pentagon to scale back "inappropriate and hostile" military drills with South Korea, hours before the exercises began on Monday.

Trump cited his "good relationship" with North Korean leader Kim Jong Un and said he was unhappy about the drills that are designed to prepare for a possible attack by Pyongyang.

South Korea's presidential office said it remained in "close coordination" with Washington on the exercises, while the defence ministry said the manoeuvres had kicked off as "previously notified and scheduled".

"Based on my very good relationship with Kim Jong Un, of North Korea, I am not happy with the fact that the United States has, long ago, agreed to participate in Joint Military Exercises with South Korea," Trump wrote on his Truth Social platform.

"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," he added.

As it was too late to cancel the drills, Trump said he had ordered Defence Secretary Pete Hegseth to "substantially reduce" them.

North Korea has not responded publicly to the statements, though it has consistently voiced anger at the regular exercises.

The 11-day manoeuvres come as Pyongyang and Moscow have grown closer, with the North providing military support -- including ground troops -- for Russia's war against Ukraine.

Colonel Ryan Donald, spokesman for the US and South Korean Combined Forces Command, had said the drills would simulate combat against North Korean troops battle-hardened by deployment in the Russia-Ukraine war.

South Korea's presidential office said in a statement to AFP that Seoul and Washington "have maintained close coordination on maintaining a firm combined defence posture and on their joint exercises and training, and will continue to coordinate closely".

It said it hoped the personal chemistry between Trump and Kim would bring them back to talks.

The office "hopes that the friendly relationship between the leaders of the United States and North Korea will lead to meaningful dialogue", it said.

- 'New level of deterrent' -

Pyongyang decries the drills as dress rehearsals for an invasion.

After test-firing a ballistic missile over the Sea of Japan as a sign of its opposition to the exercises, Pyongyang warned on Friday it could respond with a "new level of deterrent".

The US stations about 28,500 troops in South Korea to bolster the country's defences against military threats from Pyongyang, a legacy of the 1950-53 Korean War that ended with an armistice rather than a peace treaty.

But Trump has repeatedly challenged long-standing alliances, especially as Western partners have withheld overt backing for his war against Iran.

Trump took aim at South Korea in Sunday's post, writing: "I recently asked the President of South Korea if they would like to join us in the Denuclearization of the Islamic Republic of Iran, and they said, 'No thanks!'"

- 'Damage coordination' -

The exercise -- comprising 18,000 South Korean troops and counterparts from US forces in the country -- will have a heightened focus on drone warfare, GPS disruption and the role of cyber attacks, Colonel Donald had said.

Downsizing it will "damage alliance coordination, slow the process of South Korea taking on greater responsibility for its own defence, and weaken deterrence against possible conflicts in Asia", Leif-Eric Easley, a professor of international studies at Ewha Womans University in Seoul, told AFP.

"The cost savings from downsizing drills are marginal, while the diplomatic benefits for relations with North Korea are doubtful," he added.

Lim Eul-chul, a North Korea expert at Kyungnam University, told AFP Pyongyang was unlikely to be moved by Trump's announcement.

"The North will only view it as meaningful if the joint exercises are cancelled in their entirety rather than scaled back," he said.

"I am doubtful it will draw an immediate reaction from Kim Jong Un in that regard."

- 'Most hostile state' -

Experts say North Korea has expanded its nuclear and missile capabilities since the breakdown of Trump's negotiations with Kim, whom he last met in person in 2019.

North Korea has sent troops and munitions for Russia's war in Ukraine, in exchange for financial support, food, energy and military technology, according to analysts.

Kyiv recently said that up to 50,000 North Korean troops are due to deploy to Russia, although it has provided no evidence to back the claim or a timeframe for the deployment.

Ukrainian President Volodymyr Zelensky, in making the claim, asked Seoul to supply air defence systems to protect against Russian missiles, a request that has gone unanswered by the South.

South Korea's President Lee Jae Myung has reversed his predecessor's hawkish stance on the North since taking power in June 2025, offering Pyongyang talks without preconditions and the prospect of formally ending the Korean War.

But the leadership of North Korea has so far only vowed to accelerate its armament build-up, with Kim branding the South the "most hostile state".