Fresh reports have emerged of missile-related activity in North Korea, hours after Donald Trump said he would be “very disappointed” if separate reports about a rocket launch site being rebuilt proved to be true.

On Thursday South Korean media reported that cargo vehicles had recently been spotted moving around a factory at Sanumdong in Pyongyang. The site previously produced the first intercontinental ballistic missiles (ICBMs) capable of reaching the US.

South Korea’s spy chief, Suh Hoon, said he viewed the activity as missile-related, the JoongAng Ilbo newspaper reported.

Earlier, two US thinktanks and South Korea’s Yonhap news agency reported that work was under way to restore part of the Sohae satellite launching station even as Trump met Kim in Hanoi last week for their second summit.

The head of the US Indo-Pacific command, Adm Phil Davidson, said the US was keeping a close watch on North Korea. He said he was committed to maintaining UN sanctions against North Korea and a “readiness of our forces there”.

Davidson said he was working with countries including South Korea, Japan, Australia, New Zealand, Canada and France to catch any sanctions breaches via ship-to-ship transfers or other methods. “Many of those nations will contribute either maritime patrol aircraft or ships later this year. They’re all sequencing them into their schedules,” he said.

On Wednesday Trump said of the reported work at the Sohae rocket launch site: “I would be very disappointed if that were happening. 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.”

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 very closely.

North Korea began work to dismantle a missile engine test stand at Sohae in 2018 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 Hanoi 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, while adding in apparent reference to Kim: “The relationship is good.”

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. In 2018 he went as far 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 that structures on the Sohae launchpad had been rebuilt sometime between 16 February and 2 March, Reuters reported.

Satellite images suggest North Korea’s rebuilding of the Sohae launch facility. Photograph: Digitalglobe/EPA

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

News of the work at Sohae was first reported by Yonhap, which quoted South Korean lawmakers on details of a briefing by the country’s national intelligence service.

On Wednesday Yonhap quoted Suh Hoon, chief of the national intelligence service, as saying uranium enrichment facilities at North Korea’s Yongbyon nuclear complex continued to operate normally during the Vietnam summit.

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

Some analysts have interpreted the work at Sohae as an attempt by North Korea to put pressure on Washington to agree to a deal rather than as a definite move to resume tests there.