President Trump said early Thursday that he is looking forward to meeting again soon with Kim Jong Un, as the White House disclosed that Trump had received a new letter this week from the North Korean leader.

Trump expressed his interest in another meeting in a tweet posted after midnight as the U.S. military flew home what are believed to be the remains of more than 50 service members after the first such handover by North Korea in more than a decade.

“Thank you to Chairman Kim Jong Un for keeping your word & starting the process of sending home the remains of our great and beloved missing fallen!” said on Twitter. “I am not at all surprised that you took this kind action. Also, thank you for your nice letter - l look forward to seeing you soon!”

During a briefing later Thursday, White House press secretary Sarah Huckabee Sanders said, “there is not a second meeting that is currently locked in or finalized” but Trump is open to it.

The Post's Joby Warrick talks you through two of the satellite images that experts say prove North Korea continues to build its nuclear arsenal. (Jason Aldag/The Washington Post)

The return of the Korean War remains marks the first tangible move from agreements reached between Trump and Kim at a summit in Singapore on June 12 that Trump has hailed as a major success.

But in the weeks since Trump declared on Twitter that Pyongyang was “no longer a Nuclear Threat,” there have been multiple signs that call into question its commitment to denuclearizing the Korean Peninsula.

The Washington Post reported this week that U.S. spy agencies are seeing signs that North Korea is constructing new missiles at a factory that produced the country’s first intercontinental ballistic missiles capable of reaching the United States, according to officials familiar with the intelligence.

Newly obtained evidence, including satellite photos taken in recent weeks, indicates that work is underway on at least one and possibly two liquid-fueled ICBMs at a large research facility in Sanumdong, on the outskirts of Pyongyang, according to the officials, who spoke on the condition of anonymity to describe classified intelligence.

Several hours after Trump’s tweet on Thursday, the White House said the letter mentioned by the president arrived on Wednesday.

“The ongoing correspondence between the two leaders is aimed at following up on their meeting in Singapore and advancing the commitments made in the US-DPRK joint statement,” White House press secretary Sarah Huckabee Sanders said, referring to a document released by the two nations following the Singapore summit.

At the briefing, Sanders said Trump had prepared a letter to send back to Kim that should be delivered soon.

In mid-July, Trump released an earlier personal letter from Kim praising the “epochal progress” since their summit in Singapore that also suggested his desire for another meeting with Trump.

Ellen Nakashima and Joby Warrick contributed to this report.