US President Donald Trump has said he would be disappointed if North Korea were to resume weapons testing.
Mr Trump also reiterated his belief in his good relationship with North Korean leader Kim Jong Un, despite the collapse of a summit between the pair last week.
"I would be surprised in a negative way if he did anything that was not per our understanding. But we'll see what happens," Mr Trump told reporters. "I would be very disappointed if I saw testing."
Mr Trump's comments came after two US think tanks and South Korea's spy agency said this week that North Korea was rebuilding a rocket launch site at Sohae in the west of the country.
There have also been reports from South Korea's intelligence service of new activity at a factory at Sanumdong, near Pyongyang, that produced North Korea's first intercontinental ballistic missiles capable of reaching the United States.
On Friday, US National Public Radio quoted experts from California's Middlebury Institute of International Studies as saying that satellite images of Sanumdong taken on 22 February and yesterday suggested North Korea could be preparing to launch a missile or a space rocket.
One of the experts, Jeffrey Lewis, told Reuters the activity at the two sites was "probably connected."
NPR said the photos from 22 February showed cars, trucks, rail cars and two cranes at Sanumdong, while in those taken yesterday, the activity had died down and one of the cranes had disappeared.
Other experts, including Joel Wit at 38 North and Michael Elleman of the International Institute for Strategic Studies considered the conclusion speculative.
"In the past there have been multiple reports about activity at this place that turned out to be false alarms," Mr Wit said, referring to Sanumdong.
"It could either be preparation for an eventual launch or not."
The White House and the Pentagon did not immediately respond to requests for comment. The State Department declined to comment.
North Korea has frozen nuclear and missile testing since 2017, and Mr Trump has pointed to this as a positive outcome from nearly a year of high-level engagement with North Korea.
Sohae has been used in the past to test missile engines and to launch rockets that US officials say have helped development of North Korea's weapons programs.
A senior US State Department official said on Thursday that any launch from there would be "inconsistent" with North Korean commitments.
Mr Kim pledged at a first summit with Mr Trump in Singapore in June that the engine test site and launch platform at Sohae would be dismantled. He repeated the pledge in a summit with the South Korean president in September.
Mr Trump said he thought his and the US relationship with Mr Kim and North Korea was "a very good one."
"I think it remains good," he said.
Mr Trump has been eager for a big foreign policy win on North Korea which has eluded his predecessors for decades and has repeatedly stressed his good relationship with Mr Kim.
He went as far late last year as saying that they "fell in love," but the bonhomie has failed to bridge the wide gap between the two sides and a second summit between them collapsed last week in Vietnam over differences on US demands for Mr Kim to give up his nuclear weapons and North Korea's demands for sanctions relief.
US National Security Adviser John Bolton and other US officials have sought to play down the developments spotted at Sohae, although Mr Trump on Thursday called recent North Korean activity "disappointing."
The senior State Department official who briefed reporters in Washington on Thursday said he would "not necessarily share the conclusion" of the think tanks that the Sohae site was operational again, but said any use of it would be seen as "backsliding" on commitments to Mr Trump.
