North Korea's Kim Jong Un continued his official visit to Russia on Friday, stopping to inspect a fighter jet factory that is under Western sanctions due to the war in Ukraine.
The visit comes just days after Kim and Russian President Vladimir Putin held a rare summit that stoked Western concerns. There is unease that a revived Moscow-Pyongyang axis could strengthen Russia's military in Ukraine and bolster Pyongyang's missile program.
During Wednesday's talks, Kim and Putin discussed military matters, the war in Ukraine and Russian help for North Korea's satellite program.
On Friday, South Korea and the United States said that military cooperation between the two countries was a violation of United Nations sanctions and that the allies would ensure there is a price to pay.
But it was not immediately clear what - if any - leverage the United States and its Asian allies would have over either Russia or North Korea, which both have close ties to China.
In Russia's Far East, the 39-year-old North Korean leader was shown on Russian state television carefully inspecting the cockpit of a fighter jet as Russian officials explained its capabilities via a translator.
Russia has gone out of its way to publicize the visit and drop repeated hints about the prospect of military cooperation with North Korea, which was formed in 1948 with the backing of the Soviet Union.
For Putin, who says Russia is locked in an existential battle with the West over Ukraine, courting Kim allows him to needle Washington and its Asian allies while potentially securing a supply of artillery for his war in Ukraine.
Putin also could give Kim access to some of Russia's sensitive missile and other technology.
Washington has accused North Korea of providing arms to Russia, which has the world's biggest store of nuclear warheads, but it is unclear whether any deliveries have been made. Pyongyang and Moscow have denied that North Korea would supply arms to Russia.
U.S. and South Korean officials have called on Moscow to show responsibility as a permanent member of the U.N. Security Council.
There was no comment from either Kim or Putin on the U.S. warnings, although Russian diplomats pushed back against the criticism. The Kremlin says that it abides by U.N. sanctions, but that it has a right to develop neighborly relations, including on sensitive topics.