Trump's comments came after two United States think tanks and Seoul's spy agency said this week that North Korea was rebuilding a rocket launch site at Sohae in the west of the country.
The March 9 report was based on exclusive access to satellite photographs of North Korea's Sanumdong site, a facility that has been used by Pyongyang for the production of intercontinental missiles and space rockets in the past.
This week's satellite images, coming from several U.S. think tanks and testimony from the South Korean intelligence service, appear to show rapid progress has been made in rebuilding structures on the rocket launch pad.
The images appear to show activity such as cars and trucks moving around the facility, as well as cranes being erected.
The official also confirmed that Washington would seek from Pyongyang "clarifications on the purposes" of rebuilding a long-range rocket launch site, adding that so far the USA has not reached "any specific conclusion about what's happening there".
"We don't know why they are taking these steps", the official said, requesting anonymity.
After his first summit with North Korean leader Kim Jong Un in Singapore a year ago, Trump unilaterally announced the suspension of another major US-South Korea exercise, calling joint drills "very provocative" and "massively expensive".
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"Look, when I came in", he said, "under the Obama administration, North Korea was a disaster".
U.S. President Donald Trump once again reiterated that he would be "very disappointed",. if testing was resumed.
USA media had speculated over whether Trump might tighten the thumbscrews on Pyongyang following the Vietnam summit, by ratcheting up an already crippling sanctions regime.
State Department spokesman Robert Palladino affirmed Washington's commitment to remain engaged with Kim, telling journalists on Thursday that the Trump administration was ready for "constructive negotiation".
Trump said he thought his and the USA relationship with Kim and North Korea was "a very good one".
Nonetheless, "we still believe this is all achievable within the President's first term", according to the official.
Mr Trump also seemed to indicate that USA negotiations with the North Koreans could be long term.
The US is aware of the new satellite images but hasn't drawn the same conclusions as other experts, a senior official at the State Department said on Friday.
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While US officials admitted they had no clue what was happening and were seeking to "clarify" the situation, with even Bolton stressing it was too soon to make any judgement, Trump has already expressed his "disappointment" with Pyongyang's actions. "In all cases, the expectation is a complete denuclearization of North Korea as a condition for all the other steps being - all the other steps being taken".
Satellite launches use similar technology as ballistic missiles, and experts have long cautioned that North Korea's attempts to shoot satellites into space could help them develop viable long-range ballistic missiles.
A USA government source, who did not want to be identified, said North Korea's plan in rebuilding the site could have been to offer a demonstration of good faith by conspicuously stopping again if a summit pact was struck, while furnishing a sign of defiance or resolve if the meeting failed.
"All parties should have reasonable expectations on this", Wang told a news briefing.
North Korea's state media said the world is blaming the USA for ending the Hanoi summit without a deal, the first commentary from the country acknowledging the high-stakes talks failed to reach an agreement.
New satellite images have surfaced showing activity at a North Korean missile assembly site.
The development is likely to further compound Washington's frustration over the lack of progress in its bid to get the North to give up its atomic arsenal, especially after the Feb 27-28 summit between Trump and Kim collapsed without so much as a joint statement - let alone an agreement on nuclear disarmament.
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