Donald Trump has said he is scaling back joint US military drills with South Korea aimed at deterring North Korea, despite military officials from both countries warning that Kim Jong-un's regime was learning new battlefield tactics from the Ukraine war.

The drills under Ulchi Freedom Shield, one of two major annual exercises held by South Korea and the US, start on Monday and run until 27 August.

They were expected to involve about 18,000 South Korean troops, alongside US forces and personnel from 11 of the 18 member states of the United Nations Command (UNC), the multinational body that oversees the 1953 armistice that halted hostilities of the Korean war.

However, on Sunday evening, the US president said he was "not happy" with US participation "based on my very good relationship with Kim Jong-un". He added that they were costly and "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".

Trump added that it was too late to cancel the drills so he had ordered the defence secretary, Pete Hegseth, to "substantially reduce" them. He also said South Korea had refused to help in the "denuclearisation of the Islamic republic of Iran".

On Saturday, Trump also shared an old photo of him posing with the North Korean leader on Truth Social, repeating his claim the two "get along great", leading to speculation he would renew his attempt to have a summit with Kim.

Trump has previously suspended major US-South Korean military exercises in pursuit of diplomacy with Kim. After his first summit with the North Korean leader in 2018, that year's Ulchi Freedom Guardian exercise was cancelled, while other drills were suspended or scaled back. The exercises were subsequently redesigned and resumed in modified forms and names.

Cheong Seong-chang, vice president of the Seoul-based Sejong Institute thinktank, said Trump could go further if he meets Kim again, potentially deciding to suspend the exercises altogether or reduce US troop levels without consulting South Korea.

He said the US-South Korea alliance remained "extremely important" but argued that Seoul should gradually reduce its security dependence on Washington and move towards securing its own nuclear deterrent capability, an idea that has broad public support in South Korea despite being rejected by the government and opposed internationally.

However, Leif-Eric Easley, a professor of international studies at Ewha Womans University in Seoul, cautioned that downsizing the exercise would "slow the process of South Korea taking on greater responsibility for its own defence, and weaken deterrence against possible conflicts in Asia".

"The cost savings from downsizing drills are marginal, while the diplomatic benefits for relations with North Korea are doubtful," he told Agence France-Presse.

Lim Eul-chul, a North Korea expert at Kyungnam University, added that 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. It has little reason to see this as meaningful."

Last week, South Korean and US military officials said they would incorporate into the drills lessons drawn from North Korean troops' increasing battlefield experience in Ukraine, including new drone tactics.

North Korea has deepened military ties with Russia since the two countries signed a comprehensive strategic partnership pact in June 2024, seeing Pyongyang provide troops, artillery ammunition and ballistic missiles to support Moscow's war in Ukraine.

At a joint briefing, Col Ryan Donald, public affairs director for the US-led commands in South Korea, said this year's scenario would reflect "drones, GPS disruption [and] cyber attacks", threats he linked directly to North Korean soldiers who have deployed and fought in Ukraine as part of Pyongyang's military support for Russia.

"They've taken those lessons they learned there and brought them back to North Korea, that changes the DPRK capability," Donald said, referring to North Korea's formal name.

"Our training accounts for that threat, that is why the exercise looks different [from previous exercises], and over the last five years it's continually evolved."

Speaking to the Guardian at a separate briefing, Lt Gen Scott Winter, deputy commander of the UNC, said the threats being countered "reflect the increased use of these technologies and the understanding that North Korea has contemporary battlefield experience".

"We're very conscious that North Korea has used this as an opportunity to test a number of its ballistic missile systems," he said, adding that this year's drills would be "bigger and better than ever".

When asked about Trump's post, South Korea's defence ministry said the joint drills would "proceed as previously notified and scheduled".

The Guardian has reached out to South Korea's defence and foreign ministries for further comment on Trump's remarks.

More than 14,000 North Korean troops have fought alongside Russian forces in the Kursk region, and Ukraine's president, Volodymyr Zelenskyy, has warned that Russia is preparing to receive 30,000 more.

The two countries are also close to completing the first road bridge ever built between them, over the Tumen river, which one open-source investigation concluded functions more as a military logistics corridor than a trade route.

Russian state media says it is expected to open in September.

In the days before the exercise were due to begin, North Korea test-fired ballistic missiles, in what analysts read partly as a protest against the drills.

On Friday, North Korea's foreign ministry called the exercise "a rehearsal for an aggressive war", warning it would "respond to a new level of a threat with a new level of a deterrent".

The warning is typical of the rhetoric Pyongyang deploys ahead of major US-South Korean drills. Officials have repeatedly described the exercise as "defensive" in nature.

Chun In-bum, a retired lieutenant general who commanded South Korea's special forces, highlighted the importance of preparing in a timely manner for capabilities that will be required to prepare for new challenges ahead.

"This is a very serious improvement in North Korean capabilities. And we should not take lightly this threat environment."