"Amid recent satellite images hinting of activity at North Korea's missile launch facility, ".
A South Korean defense spokeswoman says the USA and South Korea are sharing intelligence about the activity at a missile research center on the outskirts of Pyongyang and at a separate long-range rocket site.
The release of the images comes after Donald Trump abruptly ended a summit with Kim Jong Un in Vietnam late last month after the USA president said the North Korean leader asked for all us sanctions to be lifted in exchange for dismantling the country's main nuclear complex.
Following the stalemate, researchers said this week that Pyongyang was rebuilding the Sohae long-range rocket site after Kim had agreed previous year to shut it as part of confidence-building measures.
North Korea's state media on Friday acknowledged for the first time that the summit between Trump and Kim Jong-un last week in Vietnam ended without an agreement.
Kim pledged at a first summit with Trump in Singapore in June that the engine test site and launch platform at Sohae would be dismantled. Lewis said it was impossible to know if North Korea is preparing a military missile or a space rocket.
Trump was unable to make any progress on a concrete plan to denuclearize the Korean Peninsula, and can claim no tangible success from the summit. The U.S. Congress has begun looking into ways to bolster existing sanctions against North Korea, while the White House has begun to fix weak points in its ship-tracking system that the North has used to evade sanctions.
The official, who did not want to be identified, said the US side still saw North Korea's complete denuclearization as achievable within Trump's current term, which ends in January 2021.
The official also threw cold water on a more incremental process, despite earlier allusions to the USA being open to such an approach. "That's our goal and that's how we see these negotiations picking up momentum".
The increase in activity is around a site known as Sanumdong, where North Korea assembled most of its ballistic missiles and rockets.
"Given that construction plus activity at other areas of the site, Sohae appears to have returned to normal operational status".
Other experts, including Joel Wit at 38 North and Michael Elleman of the International Institute for Strategic Studies considered the conclusion speculative.
In February 2018, South Korea's National Intelligence Service (NIS) also attributed the theft of tens of millions of dollars in cryptocurrencies in 2017 to North Korean hackers.
When asked about the prospects of restarting operations at an inter-Korean factory park in the North Korean border town of Kaesong and South Korean tours to the North's Diamond Mountain resort, Kim told reporters on Friday that Seoul should "work on it".
Many experts think the lack of comprehensive joint exercises will eventually likely weaken the allies' military readiness if diplomacy fails and tensions with North Korea return.
North Korea has frozen its missile tests, but it would not be surprising to see tensions rise if there is no progress toward a deal.