US spy satellites have detected renewed activity at a North Korean nuclear missile factory, according to a senior US official.

Key points: The official says one image showed a truck similar to those used previously to transport ICBMs

The official says one image showed a truck similar to those used previously to transport ICBMs The White House said it would not comment on intelligence

The White House said it would not comment on intelligence The new activity comes as North and South Korean officials resume talks to ease military standoff

Photos and infrared imaging indicate vehicles moving in and out of the facility at Sanumdong, the official told Reuters on condition of anonymity because the intelligence is classified.

The Sanumdong factory built the first North Korean intercontinental ballistic missiles (ICBMs) capable of reaching the United States.

But it was not clear how advanced any new construction might be.

The Washington Post reported that North Korea appeared to be building one or two new liquid-fuelled ICBMs at the large research facility on the outskirts of Pyongyang.

The Post's report cited unidentified officials familiar with intelligence reporting.

According to the US official who spoke to Reuters, one photo showed a truck and covered trailer similar to those the North has used to move its ICBMs.

Since the trailer was covered, it was not possible to know what, if anything, it was carrying.

The White House said it did not comment on intelligence.

Sorry, this video has expired North Korea launched a series of missile tests before agreeing to ease tensions in April.

The evidence, obtained this month, is the latest to suggest ongoing activity in North Korea's nuclear and missile facilities despite talks with the United States and a June summit between North Korean leader Kim Jong-un and US President Donald Trump.

US Secretary of State Mike Pompeo told the US Senate Foreign Relations Committee last week that North Korea was continuing to produce fuel for nuclear bombs despite its pledge to denuclearise.

But he insisted the Trump administration was still making progress in its talks with Pyongyang.

Joel Wit, a former State Department negotiator and founder of 38 North, a North Korea monitoring project, said it was unrealistic to expect North Korea to stop its programs "until the ink is dry on an agreement."

That was the case with US negotiations with the Soviet Union during the Cold War, and more recently with Iran, "which continued to build more centrifuges capable of producing nuclear material even as it negotiated with the United States to limit those capabilities," Mr Wit said.

The Sanumdong factory produced two Hwasong-15 ICBMs, North Korea's longest-range missiles, but the US official noted that Pyongyang still had not tested a reliable re-entry vehicle capable of surviving a high-velocity trip through the Earth's atmosphere and delivering a nuclear warhead.

It is possible, the official said, that any new missiles the North is building may be for further testing of such vehicles and of more accurate guidance systems.

"They seem to have figured out the engines, but not all the higher-tech stuff, and that might be what this is about," the official said.

"What's more, a liquid-fuelled ICBM doesn't pose nearly the threat that a solid-fuelled one would because they take so long to fuel, and that's something we almost certainly could see in time to abort a launch, given our assets in the vicinity."

South Korean Major General Kim Do-gyun was one of those attending the border talks. ( AP: Ahn Young-joon )

The signs of new activity at the factory came as generals from the rival Koreas met at their shared border for talks meant to ease a decades-long military standoff, according to Seoul officials.

The general-level officers, including South Korean Major General Kim Do-gyun, were discussing ways to implement April's inter-Korean summit agreements on non-nuclear military issues, but no huge announcement is expected from the talks at the border village of Panmunjom.

In the second such talks since the April summit, the generals will likely discuss dropping the number of military guards at Panmunjom, withdrawing heavy weapons from the area and pulling some army guard posts away from the Demilitarised Zone, a buffer zone that separates the two countries.

Reuters/AP/ABC