Russian President Vladimir Putin and North Korea's leader Kim Jong Un shake hands during their meeting at the Vostochny cosmodrome outside the city of Tsiolkovsky, in the far eastern Amur region, Russia, Sept. 13. AP-Yonhap

The United States is closely monitoring possible arms trade between North Korea and Russia, a senior U.S. official said Thursday, after last week's summit between their leaders raised concerns of military cooperation.

During a meeting with a group of reporters in Brussels, the official also said South Korea has expressed its willingness to actively support North Atlantic Treaty Organization (NATO) member countries that assist Ukraine in its war against Russia.

"Obviously Russia is looking for weapons and is feeling the pinch from isolation and sanctions and is shopping around," the official said on condition of anonymity. "We're watching this very closely. But to us, it's a signal that they are looking for more and the DPRK could be one possible route (for) assistance."

DPRK stands for the Democratic People's Republic of Korea, the North's official name.

North Korean leader Kim Jong-un met Russian President Vladimir Putin in Russia on Sept. 13, spawning speculation over a potential arms deal that could help Moscow's war efforts in Ukraine.

The official said it remains unknown whether such a deal will take place, noting that Pyongyang has publicly said it would not send weapons to Russia and such a move would violate U.N. Security Council resolutions.

But the official said the two countries have engaged in arms deals in the recent past.

"We've seen cases in the fall of 2021, where they were actually purchasing missiles from the DPRK. And so no doubt they have that in the back of their minds," the official said.

Washington has warned against the possible military cooperation between Pyongyang and Moscow. The U.S.' top diplomat, Antony Blinken, said Wednesday the U.S. is trying to stop the North's potential arms support to Russia "wherever we can."

During the press meeting, the U.S. official welcomed South Korea's efforts to strengthen cooperation with NATO.

"We're very excited and grateful for the South Koreans for indicating that they want to move forward with this relationship," the official said. "We're very grateful for the support that they're providing not only to Poland, but other allies that are interested in backfill."

Last year, South Korean signed major deals to supply K2 battle tanks, K9 self-propelled howitzers, Chunmoo multiple rocket launchers and FA-50 light attack aircraft to Poland, as Warsaw has been boosting its arms procurement efforts after sending military aid to Ukraine.

"The South Koreans have showed up and said we can help for those of you that have provided enormous amounts of assistance to Ukraine, we've got stuff on the shelf today that we can send and deepen our relationship," the official said.

The official pointed to South Korean soldiers at a live-fire exercise in Poland last weekend as an example of the growing cooperation between NATO and South Korea.

"Standing next to South Korean soldiers at this live-fire exercise ... was quite a moment and understanding how the world is changing," the official said, without elaborating on the details of the exercise.

The official also said South Korean officials have given briefings to NATO officials on disinformation and other security topics amid efforts to strengthen ties.

"They frequently will or regularly will brief allies on how Korea grapples with disinformation or how South Korea is protecting its technological edge, grappling with hybrid tactics from China," the official said, adding that NATO members find such briefings "interesting" as they often face the same tactics from Russia.

In July, South Korean President Yoon Suk Yeol and NATO Secretary General Jens Stoltenberg established a new bilateral partnership for cooperation in 11 areas ranging from antiterrorism to cyber defense on the sidelines of the NATO summit in Vilnius.

Although it is not a member of the alliance, South Korea was invited to the summit along with Australia, Japan and others. (Yonhap)