WASHINGTON — President Trump declared himself a “little disappointed” on Thursday by new evidence that North Korea was restoring a space-missile launch site, as a senior American official warned that a satellite launch would violate Kim Jong-un’s commitments to suspend missile and nuclear testing.

A week after a fruitless meeting in Vietnam with Mr. Kim, the North Korean leader, Mr. Trump attempted to avoid sounding very concerned about satellite evidence that Pyongyang was reactivating the launch site. Analysts believe that restoring the site is one of several steps Mr. Kim is taking to gain leverage as Mr. Trump demands a complete dismantlement of North Korea’s nuclear and missile programs.

In brief comments in the Oval Office, Mr. Trump told reporters that “we’ll let you know in about a year” whether his diplomacy in North Korea has been successful.

Last June, after his first meeting with Mr. Kim in Singapore, the president suggested that the success of the effort could be judged in six months.