South Korean and Japanese officials react to Trump’s decision, as analysts warn of weakened US alliance in the region.

The presence of US troops in South Korea is not subject to negotiations between Pyongyang and Washington, as they are a matter for the alliance between United States and South Korea, a senior official in the presidential office in Seoul said.

The unnamed official issued the statement on Friday, even as Seoul acknowledged that there have been discussions with the US about the possibility of suspending decades of joint military exercises, which Pyongyang considers as a provocative action against North Korea.

“President Moon Jae-in has already stated his position with regard to Korea-US joint exercises, and the National Security Council said it’s going to consult closely with the US based on that,” the senior official was quoted by Yonhap news agency as saying.

“In accordance with the guidelines, Korea-US discussions have already begun,” he said.

US President Donald Trump had said on Tuesday that he would stop “expensive, provocative” military exercises with South Korea, following his historic summit in Singapore with North Korean leader Kim Jong-un.

Trump also stated during his news conference in Singapore that he wants to draw down American troops in the Korean Peninsula at some point in the future, although not as a part of the negotiations with Kim.

Trump’s statements initially caught US allies South Korea and Japan by surprise, according to news reports. There have also been reports that Trump failed to inform the US defence department about his decision, before making the announcement public.

On Thursday, South Korea’s Moon expressed his willingness to suspend the joint military drills, saying that it is necessary to “flexibly change military pressure” on North Korea if it implements denuclearisation measures.

‘Pillar of peace and stability’

In Tokyo on Friday, Japanese Defence Minister Itsunori Onodera reiterated that there was no change in Japan’s stance on joint military training with US and South Korean forces.

At a press conference, Onodera said defence cooperation between Japan, the US and South Korea “is an important pillar to ensure regional peace and stability”.

Meanwhile, former US officials and defence analysts said Trump’s decision on joint military exercises could weaken allied defenses in the Asia-Pacific, and potentially damage diplomatic relations in the region.

“Those exercises are critically important because they are deterrence,” said Chuck Hagel, a former defence secretary in administration of former US President Barack Obama.

Hagel, a former Republican senator, welcomed Trump’s willingness to talk to Kim but worried that the president has underestimated the complications he has introduced for the Pentagon by suspending the military drills.

“You don’t just shut them on and off like a water faucet,” he said.

The US has stationed combat troops in South Korea since the Korean War ended in 1953 with an armistice and no peace treaty.

Trump’s ‘dubious’ assertion

Currently, there are an estimated 28,000 US forces in South Korea.

Annual large-scale exercises in the Korean Peninsula are done to ensure that evolving tactics, procedures and plans can be carried out smoothly, and that US and South Korean forces are in sync.

They also are a means of showing allied solidarity, which is part of the psychology of deterring enemy attack.

The next major exercise with South Korea is known as Ulchi Freedom Guardian; last year’s version was held for 11 days in August and involved about 17,500 US troops.

Michael Green, who was Asia director on the National Security Council staff during the George W Bush administration, said the likely damage from suspending drills is multiplied by Trump’s failure to inform South Korean and Japanese officials in advance and his focus on cost-savings.

This was then compounded, in Green’s view, by Trump’s dubious assertion on Twitter that North Korea no longer poses a nuclear threat.

“The number one problem with this, geopolitically, is that it suggests to our allies that we are just incompetent, that we don’t recognise the threat,” Green told the Associated Press news agency.

Christine Wormuth, the Pentagon’s top policy official from 2014 to 2016, also criticised Trump’s statement likening it to “North Korean and Chinese talking points”.