A U.S. think tank specializing in relations between North Korea and the Western world said Wednesday that it had detected activity at one of North Korea’s nuclear enrichment sites.

The Washington-based Center for Strategic and International Studies told Reuters on Tuesday that satellite images showed specialized rail cars that could be used to transport radioactive materials outside the Yongbyon nuclear site in North Korea last week, indicating that the facility may be in use.

“In the past these specialised railcars appear to have been associated with the movement of radioactive material or reprocessing campaigns,” the report obtained by Reuters reads. “The current activity, along with their configurations, does not rule out their possible involvement in such activity, either before or after a reprocessing campaign.”{mosads}

Yongbyon was not one of the nuclear sites covered under an initial agreement between President Trump and Kim Jong Un last year for North Korea to dismantle its nuclear sites, but was discussed at recent summit between the two leaders in February. That second summit in Hanoi, Vietnam, ended without an agreement.

“Because there wasn’t an agreement with North Korea on Yongbyon, it would be interesting timing if they were to have started something so quickly after Hanoi,” said Jenny Town, an expert with the Stimson Center, according to Reuters.

Kim indicated this week that the U.S. had until the end of the year to reach a second agreement with North Korea on the issue of denuclearization and sanctions relief, while opening the door to a third summit between himself and Trump.

“It is essential for the U.S. to quit its current calculation method and approach us with a new one,” Kim told the North Korean People’s Assembly, according to Reuters.

The president responded to Kim’s remarks on Twitter, claiming that “our personal relationship remains very good,” while adding that “a third Summit would be good in that we fully understand where we each stand.”

“I look forward to the day, which could be soon, when Nuclear Weapons and Sanctions can be removed, and then watching North Korea become one of the most successful nations of the World!” Trump added.