South Korea's Yonhap quoted lawmakers briefed by the country's National Intelligence Service (NIS) as saying that the work was taking place at the Tongchang-ri launch site and involved replacing a roof and a door at the facility.

Satellite images seen by 38 North, a Washington-based North Korea project, showed that structures on the launch pad had been rebuilt sometime between February 16 and March 2, Jenny Town, managing editor at the project and an analyst at the Stimson Center think tank, told Reuters.

The news comes days after a second summit on denuclearisation between Trump and North Korean leader Kim Jong-un broke down over differences on how far North Korea was willing to limit its nuclear program and the degree of US willingness to ease sanctions.

The summit took place in Hanoi on February 27 and 28.

Trump told a news conference after an unprecedented first summit with Kim on June 12 in Singapore that the North Korean leader had promised that a major missile engine testing site would be destroyed very soon.

Trump did not identify the site, but a US official subsequently told Reuters it was the Sohae Satellite Launching Ground, which is located at Tongchang-ri.

A US government source said the NIS was considered reliable on such issues, but added that the work described did not seem particularly alarming, and certainly not on a scale of resuming missile tests that have been suspended since 2017.

Kim Jong-un also pledged at a summit with South Korean President Moon Jae-in in September to close Sohae and allow international experts to observe the dismantling of the missile engine-testing site and a launch pad.

Signs that North Korea had begun acting on its pledge to Trump were detected in July, when a Washington think tank said satellite images indicated work had begun at Sohae to dismantle a building used to assemble space-launch vehicles and a nearby rocket engine test stand used to develop liquid-fuel engines for ballistic missiles and space-launch vehicles.

However, subsequent images indicated North Korea had halted work to dismantle the missile engine test site in the first part of August.

The breakdown of the summit in Hanoi last week has raised questions about the future of US-North Korea dialogue.

US Secretary of State Mike Pompeo on Monday said he was hopeful he would send a delegation to North Korea in the coming weeks but that he had had "no commitment yet."

State Department spokesman Robert Palladino told a news briefing that the US remains "in regular contact" with North Korea.

Yonhap quoted lawmakers briefed by intelligence officials as saying that the five-megawatt reactor at North Korea's main nuclear site at Yongbyon, which produces weapons-grade plutonium used to build bombs, had not been operational since late last year, concurring with a report from the UN atomic watchdog.

Yonhap quoted the sources as saying there had been no sign of reprocessing of plutonium from the reactor and that tunnels at North Korea's main nuclear test site in Punggye-ri had remained shut down and unattended since their widely publicised destruction in May.

Australian Associated Press