Mike Pompeo will travel to North Korea on Thursday in an effort to press the Pyongyang regime on commitments the US said it gave at a summit last month in Singapore, the state department has confirmed.

In preparation for the two-day trip, US and North Korean officials are reported to have met in the demilitarised zone between the two Koreas over the weekend, amid reports that Pyongyang is stepping up its nuclear and missile programmes since last month’s Singapore summit.

Andrew Kim, the head of the CIA’s Korea department and Sung Kim, a veteran negotiator who is now US ambassador to the Philippines, met North Korean counterparts at Panmunjom in the DMZ, according to South Korea’s Chosun Ilbo newspaper.

The state department said on Monday that Pompeo would fly to Pyongyang on Thursday and stay until Saturday, to “continue consultations and implement the forward progress made by President Trump and Chairman Kim in Singapore”.

Pompeo will then travel to Tokyo, where he will meet the Japanese and South Korean leaders “to discuss our shared commitment to the final, fully verified denuclearization of the DPRK, as well as other bilateral and regional issues”, the state department said.

After the Singapore summit, Trump tweeted that: “There is no longer a nuclear threat” from North Korea, but in recent days, multiple leaks from the US intelligence assessment have suggested that the regime’s work on its nuclear and missile programme is not just continuing, but accelerating.

The Wall Street Journal reported on Sunday that North Korea had expanded a factory for manufacturing solid fuel ballistic missiles in Hamhung, based on satellite imagery analysed by the Middlebury Institute of International Studies (MIIS) at Monterey, California.

Satellite image of a North Korean missile production facility in the city of Hamhung. Photograph: Handout/Reuters

The Diplomat cited intelligence officials as saying Pyongyang was also continuing work on mobile launchers for one its latest generation of ballistic missiles.

This follows a report by NBC News, also quoting multiple intelligence officials, which said work had been stepped up at secret uranium enrichment sites.

North Korea has acknowledged running one enrichment plant at its nuclear complex at Yongbyon. However, in 2010 US intelligence found a site at a place called Kangson which it believes to be a covert parallel site. A third site is said to have been discovered more recently.

Officials from the Defence Intelligence Agency were quoted by CNN as saying they believe Kim has no intention of disarming, at least for the time being, and would seek to hide much of its programme.

Pompeo is expected to present the Pyongyang leadership with a demand for a full inventory of its nuclear and missile programmes and then a timetable for dismantling them.

Speaking to CBS News on Sunday, the national security adviser, John Bolton, said the US had a plan for full disarmament within a year.

“I’m sure that secretary of state Mike Pompeo will be discussing this with the North Koreans in the near future about really how to dismantle all of their WMD and ballistic missile programs in a year,” Bolton told the programme Face the Nation.

Most observers are sceptical that pace of disarmament is feasible even if Kim was willing to disarm, which they doubt.

“It is unhelpful when Trump administration officials set arbitrary timelines for the denuclearisation process, which is going to be a long-term process that is going to require reciprocal US actions,” Kelsey Davenport, the director for non-proliferation policy at the Arms Control Association in Washington, said. “The speed at which North Korea is willing to take steps is dependent on what the US puts on the table in return.”

Experts express doubt that Kim Jong-un is willing to disarm. Photograph: AFP/Getty Images

“Kim has not said he would disarm at any point in this process,” said Jeffrey Lewis, the director of the East Asia Nonproliferation programme at MIIS. In Singapore, Kim signed a joint statement with Trump committed to “complete denuclearisation of the Korean peninsula”, but that has been the North Korean position since 1992 and does not necessarily mean the regime is ready to disarm.

“What it means to them is a formal process in which the US give up its nuclear weapons and then North Korea goes along,” Lewis said. “What we want is disarmament but we started using their phrase in the misguided belief it would cause them to get confused and give up their weapons by mistake.”

Jim Walsh, an expert on the North Korean nuclear programme at the Massachusetts Institute of Technology, said that North Korea’s total disarmament is the wrong benchmark to judge the success of diplomatic engagement.

“Hey, I’d love to have denuclearisation. But it’s the wrong frame,” Walsh said. “The issue is whether we are safer or more insecure. Is a conflict that could lead to the use of nuclear weapons more likely or less likely?”