Movement of cargo vehicles also spotted recently around an intercontinental ballistic missile plant, news reports say.

US President Donald Trump said he would be “very disappointed” in North Korean leader Kim Jong Un if reports about rebuilding at a rocket launch site in North Korea proved true.

Trump’s comments came as new activities were detected at a North Korean intercontinental ballistic missile plant, South Korean media reported on Thursday.

Movement of cargo vehicles was spotted recently around a factory at Sanumdong in the capital Pyongyang, which produced North Korea’s first intercontinental ballistic missile capable of reaching the United States, South Korea’s JoongAng Ilbo and Donga Ilbo newspapers reported, citing legislators briefed by the National Intelligence Service (NIS).

Spy chief Suh Hoon told the politicians on Tuesday he viewed the activity as missile-related, according to JoongAng Ilbo.

The newspaper also quoted Suh as saying North Korea continued to operate its uranium-enrichment facility at the main Yongbyon nuclear complex. But that contradicted reports from a day earlier that no activity was ongoing there since late last year, concurring with findings from the UN atomic watchdog.

‘Rocket man’

Two US think-tanks and South Korea’s Yonhap News Agency reported work was under way to restore part of North Korea’s Sohae Satellite Launching Station – even as Trump met Kim at a second summit in Hanoi last week.

“I would be very disappointed if that were happening,” Trump told reporters when asked if North Korea was breaking a promise.

“Well, we’re going to see. It’s too early to see … It’s a very early report. We’re the ones that put it out. But I would be very, very disappointed in Chairman Kim – and I don’t think I will be – but we’ll see what happens. We’ll take a look. It’ll ultimately get solved.”

North Korea began work to dismantle a missile engine test stand at Sohae last year after pledging to do so in a first summit with Trump in June.

A second summit between Trump and Kim broke down last week in Vietnam over differences on how far North Korea was willing to limit its nuclear programme and the degree of US willingness to ease sanctions.

“We have a very nasty problem there. We have to solve a problem,” Trump said on Wednesday, adding in apparent reference to Kim: “The relationship is good.”

Loving Kim

Trump – eager for a big foreign policy win on North Korea that has eluded predecessors for decades – has repeatedly stressed his good relationship with Kim. He went as far late last year as saying they “fell in love”, but the bonhomie has failed so far to bridge the wide gap between the two sides.

Satellite images seen by 38 North, a Washington-based North Korea project, showed structures on the Sohae launch pad had been rebuilt sometime between February 16 and March 2.

The Center for Strategic and International Studies think-tank released a separate report, also citing satellite imagery, that concluded North Korea was “pursuing a rapid rebuilding” at the site.

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News of the work at Sohae was first reported by Yonhap, which quoted South Korea legislators on details of the NIS briefing.

The White House did not immediately respond when asked what Trump meant by “we’re the ones that put it out”, but US and South Korean intelligence agencies cooperate closely.

The reasons for construction at the launch site remain unclear.

Some analysts say the work is a signal that Kim is getting ready to conduct more tests, but others suggest he is just registering his disappointment that no agreement was reached at the summit.

“The North Koreans actually haven’t used it formally for missile tests and it’s not necessarily clear the rebuild is actually leading up to that,” Robert Kelly, a professor at Pusan National University, told Al Jazeera.

“My own sense is this is sort of pique by the way Donald Trump treated them in Vietnam.”

Joel Wit, a North Korea proliferation expert who helped negotiate with North Korea in the mid-1990s, said the Sohae reports are Kim’s way of showing he’s “getting impatient with lack of progress in negotiations”.

“We have to watch to see what else happens,” Wit said. “It’s a space launch facility and has been used to send satellites into space … Problem is some of the technologies [with rockets] are the same.”

Sanctions warning

Trump’s national security adviser, John Bolton, warned on Tuesday new sanctions could be introduced if North Korea did not scrap its nuclear weapons programme.

The summit breakdown and sanctions threat have raised questions about the future of denuclearisation dialogue.

While North Korea’s official media said last week Kim and Trump decided at the summit to continue talks, its vice foreign minister, Choe Son Hui, told reporters Kim “might lose his willingness to pursue a deal” and questioned the need to continue.

White House spokeswoman Sarah Sanders said on Wednesday the United States was “continuing to have ongoing conversations with North Korea” but did not elaborate.