The official also confirmed that Washington would seek from Pyongyang "clarifications on the purposes" of rebuilding a long-range rocket launch site, adding so far the US has not reached "any specific conclusion about what's happening there."

The specialized website 38 North and the Center for Strategic and International Studies used commercial satellite imagery to track construction at the site -- which they said began before last week's aborted summit in Hanoi between Trump and North Korea's Kim Jong Un.

Images taken on March 6 showed that a rail-mounted structure to transfer rockets to the launching pad appeared to have been completed and "may now be operational."

Kim had agreed to shutter the Sohae Satellite Launching Station at a summit with the South's President Moon Jae-in in Pyongyang as part of confidence-building measures.

The official, who requested anonymity, recalled the president's statement that he would be "very, very disappointed" if reports of activity at the site were true.

"We're watching in real time as you are the developments at Sohae," he explained, adding: "We don't know why they are taking these steps."

In Hanoi, Vietnam, late last month, the US president and Kim -- meeting for the second time -- failed to make progress or even arrive at a joint statement.

Nonetheless, "we still believe this is all achievable within the president's first term," according to the official. Unless re-elected, Trump's term will end in January 2021.

"We have sufficient time," he said, without mentioning a deadline for reaching an agreement so the goal could be met. It is not clear if the US has had contact with North Korea since the Hanoi summit.

"Where we really need to see progress and we need to see it soon is meaningful and verifiable steps on denuclearization as quickly as we can," he added.

"We are mindful that every day the challenge is greater, the threat posed ... is not going away."