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The United States will ask North Korea for clarification on the purposes of its reassembly of a key missile facility, a senior U.S. official said Thursday as President Donald Trump expressed repeated disappointment over the activity.

North Korea began to rebuild parts of the missile engine testing site in Dongchang-ri in the runup to Trump's second summit with North Korean leader Kim Jong-un in Hanoi, Vietnam, on Feb. 27-28, according to two U.S. think tanks earlier this week.

By Wednesday, commercial satellite imagery showed that the facility may have been restored to normal operational status following its partial dismantlement last year, they said.

Trump said Wednesday he would be "very, very disappointed" with Kim if the reports were true, and on Thursday added he was "a little disappointed" by Kim and the reported activity.

"We don't know why they are taking these steps," the State Department official told reporters on background.