Satellite images published on Wednesday suggest Pyongyang is restoring a site it had pledged to scrap.
Work to dismantle the Sohae satellite launch site began last year and was seen as a concession by Pyongyang.
Talks between Mr Trump and North Korean leader Kim Jong-un broke down at a summit in Vietnam last week.
Speaking to reporters, Mr Trump said it was too early to tell if the information was true.
“I would be very disappointed if that were happening,” Mr Trump said on Wednesday in response to the reports about the launch site.
“It’s a very early report. I would be very, very disappointed in Chairman Kim – and I don’t think I will be – but we’ll see what happens. We’ll take a look. It’ll ultimately get solved.”
A much anticipated meeting between the two leaders in the Vietnamese capital last week ended without a deal over differences in how much North Korea was willing to limit its nuclear programme before it was granted some sanction relief.
The Sohae launch facility at the Tongchang-ri site has been used for satellite launches and engine testing but never for ballistic missile launches.
The new satellite images, coming from several US think tanks and testimony from the South Korean intelligence service, appears to show rapid progress has been made in rebuilding structures on the rocket launch pad.
However, the BBC’s Laura Bicker says the renewed activity may just be Pyongyang’s way of sending a reminder to Washington that it has the technology to build weapons.
US National Security Adviser John Bolton earlier said North Korea could yet face more sanctions if there was no progress on denuclearisation.
A historic first meeting between the two in 2018 in Singapore produced a vaguely worded agreement on “denuclearisation” but little progress.