Movement of cargo vehicles was spotted recently around a factory at Sanumdong in Pyongyang, which produced North Korea's first intercontinental ballistic missiles capable of reaching the US, South Korea's JoongAng Ilbo and Donga Ilbo newspapers reported on Thursday, citing lawmakers briefed by the National Intelligence Service.

Spy chief Suh Hoon told the lawmakers he viewed the activity as missile-related, the JoongAng Ilbo said.

It quoted Suh as saying North Korea continued to run its uranium enrichment facility at the main Yongbyon nuclear complex after the first summit between Trump and its leader, Kim Jong-un, in June in Singapore.

The reports came after the leaders' second summit, in the Vietnamese capital of Hanoi, broke down last week over differences on the limits North Korea was ready to put on its nuclear program and how willing the US was to ease sanctions.

White House national security adviser John Bolton said on Thursday that Trump was open to additional talks with North Korea over denuclearisation.

"The president's obviously open to talking again. We'll see when that might be scheduled or how it might work out," Bolton said in an interview with Fox News.

Bolton also said it was too soon to make a determination on the reports of North Korea's missile activities.

"We have a lot of ways of getting information," he said. "We're going to study the situation carefully. As the president said, it would be very, very disappointing if they were taking this direction."

South Korea's presidential office and defence ministry declined to confirm the reports on Sanumdong, saying they were closely monitoring North Korea's activities together with the United States.

The US State Department said it could not comment on intelligence matters.

On Tuesday, two US think tanks and Seoul's spy agency said work was under way to restore part of the North's Sohae rocket launch site that Kim, at the Singapore summit, vowed to dismantle.

"I would be very disappointed if that were happening," Trump told reporters on Wednesday in the Oval Office, when asked if North Korea was breaking a promise.

"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."

Another US research body, Washington-based Stimson Center's 38 North, weighed in on Thursday, saying the Sohae site appeared to have returned to normal operational status after recent rebuilding work.

Commercial satellite imagery, 38 North said, "indicates construction to rebuild the launch pad and engine test stand that began before the Hanoi summit has continued at a rapid pace".

A US government source said the work at Sohae probably began before the summit, which was preceded by lower-level talks in February.

Some analysts see the work at Sohae as aimed at pressing Washington to agree to a deal, rather than as a definite move to resume tests.

Australian Associated Press