WASHINGTON — President Donald Trump said Thursday that he’s a “little disappointed” by reports of new activity at a North Korean missile research center and long-range rocket site and that time will tell if U.S. diplomacy with the reclusive country will be successful.

South Korea’s military said it is carefully monitoring North Korean nuclear and missile facilities after the country’s spy agency told lawmakers that new activity was detected at a research center where the North is believed to build long-range missiles targeting the U.S. mainland.

Defense Ministry spokesman Choi Hyun-soo said the U.S. and South Korean militaries are sharing intelligence over the developments at the North’s missile research center in Sanumdong on the outskirts of the capital, Pyongyang, and at a separate long-range rocket site. She did not elaborate on what the developments were.

Asked if he was disappointed in the new activity, Trump told reporters at the White House that he was “a little disappointed.” Then he said time will determine the future of U.S. efforts to get North Korean leader Kim Jong Un to give up his pursuit of nuclear weapons in exchange for relief from sanctions stalling economic growth.

Briefing reporters at the State Department later, a senior U.S. official said that despite the new activity and the failure of last month’s Trump-Kim summit in Hanoi to reach a denuclearization deal, the administration still believes it can reach and implement an agreement by the end of the president’s first term. The official said it is important that progress be made quickly but that the goal is “achievable” by January 2021.

The official said that the U.S. is still trying to determine exactly what North Korea is doing with recent activity but that the administration will seek clarification from the North as well as intelligence analysts. The official said the Trump administration did not necessarily agree with nongovernmental analysts who believe the activity is a sign of North Korean anger following the summit.

Trump said on Wednesday that his relationship with Kim remained “good” even though Trump walked away from negotiations at their high-profile meeting in Vietnam, saying the North’s concessions on its nuclear program weren’t enough to warrant sanctions relief.