U.S. President Donald Trump holds up a letter from former President Joe Biden while signing executive orders in the Oval Office at the White House in Washington on Inauguration Day, Monday (local time). Reuters-Yonhap

Newly inaugurated U.S. President Donald Trump referred to North Korea as a "nuclear power," Monday (local time), a remark that raised concerns in South Korea as officials brace for sweeping shifts in Washington's North Korea policy under the new administration.

The comments, made in the first few hours of his presidency, were rare for a U.S. official, let alone an incumbent president. Some analysts interpreted this as a sign that the Trump administration might be poised to officially acknowledge North Korea's de facto nuclear status.

Trump spoke about North Korean leader Kim Jong-un during an impromptu meeting with reporters in the Oval Office, shortly after his swearing-in ceremony at the Capitol Rotunda in Washington.

"I was very friendly with him. He liked me. I liked him. We got along very well," Trump said. "He is a nuclear power. We got along. I think he will be happy to see me coming back."

The remark about North Korea being a nuclear power marks a sharp departure from previous U.S. policy, as officials had long avoided publicly acknowledging Pyongyang as a nuclear-armed nation. Even Trump, who had developed a personal rapport with Kim during his first term, had refrained from making such references in his talks with the North Korean leader.

While "nuclear power" is not an official term under the Treaty on the Non-Proliferation of Nuclear Weapons, which distinguishes between nuclear-weapon states and non-nuclear-weapon states, it is noteworthy that the phrase was recently used by Trump's Defense Secretary Pete Hegseth.

During his confirmation hearing last week, Hegseth said Pyongyang's "status as a nuclear power" poses a threat to both the Korean Peninsula and global security.

North Korean leader Kim Jong-un, left, sits with U.S. President Donald Trump for dinner at a hotel in Hanoi, Vietnam, Feb. 27, 2019. AFP-Yonhap

"It is unclear whether Trump was referring to North Korea's nuclear weapons in terms of military capability and technology, or to its status as a nuclear state in political and diplomatic terms, as those two are very different concepts," Yang Moo-jin, president of the University of North Korean Studies, said.

"If he was referring to the latter, which could pave the way for U.S. support for formally recognizing North Korea as a nuclear state under international law, it would be a highly contentious move that could strain South Korea's goal of denuclearizing the Korean Peninsula."

There is increasing speculation that if Washington and Pyongyang were to resume talks, Trump may focus on securing a freeze on North Korea's nuclear weapons to reduce threats to the U.S., rather than pursuing the long-standing goal of complete denuclearization.

This could be welcomed by the Kim regime, which has long sought international recognition of its nuclear arsenal.

Lim Eul-chul, an expert on North Korea at the Institute of Far Eastern Studies, said, "It's not a coincidence that both Trump and Hegseth, who will be leading U.S. policy on North Korea over the next four years, have used this term. And it's definitely something Kim Jong-un wants to hear."

Trump may make further overtures to Kim in an effort to bring him back to the negotiation table. These gestures could include scaling back joint South Korea-U.S. military exercises or reducing the deployment of American strategic assets to the Korean Peninsula — both of which the Kim regime views as rehearsals for invasion.

Meanwhile, the new U.S. leader held a video call with U.S. Forces Korea (USFK) troops during the Commander-in-Chief Ball after his inauguration ceremony, during which he asked, "How is Kim Jong-un doing?"

He added, "I developed a pretty good relationship with him, but he's a tough cookie."

South Korean and U.S. troops participate in a joint military exercise at Rodriguez training field in Pocheon, Gyeonggi Province, Jan. 15. Yonhap

Regarding the new U.S. leader's comments, the South Korean Ministry of Unification stressed that Seoul's unified goal of complete denuclearization with Washington remains unchanged.

"South Korea and the U.S. have maintained a firm and unified position on the goal of North Korea's complete denuclearization. The government will establish a close cooperation system with the new U.S. administration," a ministry official said.

The Ministry of Foreign Affairs stressed that "the goal of denuclearizing North Korea remains a principle consistently upheld by South Korea, the U.S. and the international community," and that "North Korea can never attain the status of a nuclear-weapon state under the NPT."

Yet, potential Trump-Kim nuclear talks pose a diplomatic challenge to South Korea as the country grapples with deepening political turmoil. President Yoon Suk Yeol is under criminal investigation for his martial law imposition late last year and also faces impeachment trials, which will determine in the coming months whether he will be formally ousted.

"Considering Trump's preference for leader-to-leader exchanges, he is unlikely to engage seriously with South Korea under the acting leadership. This is a concerning development as Washington and Pyongyang may begin behind-the-scenes engagement in the coming months," Lim said.

Acting President and Finance Minister Choi Sang-mok said Tuesday that he plans to arrange phone talks with Trump in the near future, while stating his wariness of policy shifts under the new U.S. president.

Foreign Minister Cho Tae-yul is seeking to visit Washington as early as possible as his ministry works to minimize the diplomatic void with the nation's key ally during the leadership crisis.