North Korean leader Kim Jong-un has inspected a Russian fighter jet factory that is under Western sanctions as part of a visit Washington and its allies fear could strengthen Russia's military in Ukraine and bolster Pyongyang's missile program.
Russian President Vladimir Putin had discussed military matters and deepening co-operation with Kim when they met at a summit on Wednesday.
South Korea and the United States said on Friday that military co-operation between North Korea and Russia was a violation of United Nations sanctions and the allies would ensure there is a price to pay.
Kim, 39, on Friday visited aviation facilities in the far eastern city of Komsomolsk-on-Amur, the Yuri Gagarin Aviation Plant and the Yakovlev plant, both units of United Aircraft Corporation, which is sanctioned by the West over the war in Ukraine.
At the Gagarin plant, which is also specifically sanctioned by the US, Kim inspected the assembly workshops where the Sukhoi Su-35 multirole fighter and the Su-57 fighter are made, escorted by Deputy Prime Minister Denis Manturov, the government said.
Kim was shown on Russian state television carefully inspecting the cockpit of a fighter jet as Russian officials explained its capabilities via a translator.
He then inspected workshops where the fuselage compartments and wing assemblies of Russia's Sukhoi Superjet 100 are made before watching a demonstration flight of the Su-35.
He nodded with approval as the fighter performed.
Russia has gone out of its way publicise the visit and to drop repeated hints about the prospect of military co-operation 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 deep supply of artillery for the Ukraine war.
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.
Putin and Kim called each other "comrades" when they met at Russia's most advanced space rocket launch cosmodrome on Wednesday, with Kim toasting Putin's health and victory for Russia in Ukraine.
The US and South Korea appear worried by the revival of Moscow's friendship with Pyongyang which they fear could give Kim access to some of Russia's sensitive missile and other technology.
US and South Korean officials called on Moscow to show responsibility as a permanent member of the United Nations Security Council.
"We agreed to work together to ensure there is a price to pay for the grave violation of Security Council resolutions," South Korean Vice Foreign Minister Chang Ho-jin told a press conference in Seoul.
There was no comment from either Kim or Putin on the US warnings, although Russian diplomats pushed back against the criticism.
They said Washington had no right to lecture Moscow after the US had bolstered its allies across the world, including with a visit of a US nuclear-armed ballistic missile submarine to South Korea in July.
The US State Department said on Wednesday the Biden administration would not hesitate to impose additional sanctions on Russia and North Korea if they conclude new arms deals.
The Kremlin says it abides by UN sanctions but that it has a right to develop neighbourly relations, including on sensitive topics.
Kim arrived in Russia on his special train on Tuesday, held a summit with Putin on Wednesday in Vostochny and appeared to have spent most of Thursday travelling before appearing in Komsomolsk-on-Amur, 6000km east of Moscow, on Friday.
