Seoul, South Korea -- U.S. analysts say North Korea appears to have restored normal operations at a long-range rocket launch site it had partially dismantled last year as part of disarmament steps. Some experts say North Korea is trying to convey displeasure over the breakdown of a high-stakes nuclear summit last week between leader Kim Jong Un and President Trump over what the Americans said were Kim's excessive demands for sanctions relief.

The Trump administration, however, has said it doesn't necessarily agree with public analysis of what the Kim regime's renewed activity means, and it still believes its goal of "denuclearizing" North Korea is attainable during Mr. Trump's first term.

North Korea's state media on Friday acknowledged for the first time that the summit ended without an agreement. But the Rodong Sinmun, which primarily targets the domestic audience, held back from criticizing the United States and instead berated "detestable" Japan for supposedly celebrating the "unexpected" setback and supporting sanctions against the North.

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The United States and North Korea accused each other of causing the breakdown of the talks in Vietnam, but both sides left the door open for future negotiations.

North Korea-focused website 38 North said Thursday that commercial satellite images from March 6 indicate that the Sohae launch site appears to have returned to "normal operational status" following rapid construction to rebuild a launch pad and a rocket engine test stand.

A commercial satellite image taken over Tongchang-ri, North Korea on March 2, 2019 and released March 5, 2019 shows apparent rebuilding work at the Sohae Satellite Launching Station launch pad, according to researchers of Beyond Parallel, a CSIS project HANDOUT/REUTERS

The Washington-based Center for Strategic and International Studies published similar findings and said the North's actions amount to a "snapback" from the moderate dismantlement it undertook following the first Trump-Kim summit last June.

"The rebuilding activities at Sohae demonstrate how quickly North Korea can easily render reversible any steps taken toward scrapping its WMD program with little hesitation," the CSIS said in a study authored by Joseph Bermudez and Victor Cha. "This poses challenges for the U.S. goal of final, irreversible and verifiable denuclearization."

Mr. Trump has said he's a "little disappointed" by the reports of the new North Korean activity and that time will tell if U.S. diplomacy with the reclusive country will be successful.

Despite the indications that the Kim regime could be backpedalling, however, a senior State Department official said on Thursday that, in the Trump administration's view, denuclearization is still achievable during Mr. Trump's first term. The official said the administration had seen suggestions that work at Sohae may have been going on even before the Hanoi summit, but that the White House has not necessarily reached the same conclusion as the organizations offering public analysis.

The official added, however, that it would represent backsliding by North Korea if the regime was to start using the facility again in any capacity because they had agreed to dismantle it. A new satellite launch would be, in the administration's view, "inconsistent with the commitments the North Koreans have made."

The Sohae satellite launching center in Tongchang-ri, a seaside region in western North Korea, is where the North carried out satellite launches in recent years, resulting in U.N. sanctions over claims that they were disguised tests of banned missile technology.

Some experts see the North as trying to put pressure on Washington and Seoul, which has acted as a mediator, to make a deal by creating an impression that it could resume missile or rocket tests.

South Korea's spy agency has also told lawmakers in a closed-door intelligence briefing that increased vehicle movement was detected at a missile research center on the outskirts of Pyongyang where the North is believed to build long-range missiles targeting the U.S. mainland.

This undated picture released by North Korea's official Korean Central News Agency (KCNA) on March 19, 2017, shows North Korean leader Kim Jong Un (R) inspecting the ground jet test of a newly developed high-thrust engine at the Sohae Satellite Launching Ground in North Korea. Getty

South Korea's Defense Ministry said Thursday that it is carefully monitoring North Korean nuclear and missile facilities and that the U.S. and South Korean militaries were closely coordinating intelligence over the developments at Tongchang-ri and the missile research center.

Moon Seong Mook, an analyst for the Seoul-based Korea Research Institute for National Strategy, said it's unlikely that North Korea will resume major missile tests or satellite launches anytime soon because that would risk destroying its fragile negotiations with Washington and could bring even harsher sanctions on its crippled economy.

He said North Korea will also want to see if South Korea will support its position more strongly. Undeterred by the breakdown of the Trump-Kim summit, South Korea has continued to urge the United States to ease sanctions on North Korea to allow a resumption of inter-Korean economic projects and encourage more disarmament steps from the North.