South Korea's ambassador to the United States on Tuesday called the apparent strengthening of cooperation between Russia and North Korea "most concerning," while stressing Seoul and Washington will "not sit idly by."

Ambassador Cho Hyun-dong made the remarks amid concerns that a recent summit between Russian President Vladimir Putin and North Korean leader Kim Jong-un might have led to an arms deal feared to help enhance Pyongyang's military capabilities and support Russia's war operations in Ukraine.

"What is the most concerning is the strengthening of cooperation between Russia and North Korea," Cho told a press meeting as he touched on a series of recent developments that could affect the security landscape in Northeast Asia.

"While circumstantial (evidence) regarding illicit arms trade between Russia and North Korea has emerged, the international community's concerns about the two countries getting closer -- as seen in their recent summit having taken place for the first time in four years -- have been growing," he added.

Cho said that what Russia and the North seek to gain through a deal is "directly linked" to not only Ukraine's security, but also that of South Korea, as he noted that Russia is striving to secure military supplies in its war in Ukraine, with the North seeking technology to address failures in satellite launches earlier this year.

"The South Korea-US alliance will never sit idly by when it comes to any threat to our security," Cho said.

He also commented on the US Department of Commerce's recent announcement on the finalized national security "guardrails" aimed at restricting semiconductor subsidy recipients from expanding their manufacturing capacity in China and other "foreign countries of concern."

"For South Korean semiconductor companies investing in the US, uncertainties have been addressed, and we expect that it would help their normal business activities regarding factories operating in China to a certain extent," he said.

He pledged to provide necessary support for South Korean businesses regarding the guardrails while maintaining close consultation with the US government.

Last Friday, the Commerce Department announced the final guardrails of the CHIPS and Science Act, which are seen as an effort to prevent China's technological advancements that could pose security risks to the US and its allies.

It prohibits the material expansion of semiconductor manufacturing capacity for advanced facilities in China for 10 years from the date of award. The rule defined material expansion as increasing a facility's production capacity by "more than five percent." It also prohibits the expansion of production capacity for legacy facilities beyond 10 percent. (Yonhap)