South Korea, Japan and the United States "strongly condemn" North Korea supplying arms to Moscow, the allies said in a joint statement Thursday, adding that "several" such deliveries have been made by Pyongyang.
Russia and North Korea, historic allies, are both under a raft of global sanctions -- Moscow for its invasion of Ukraine, and Pyongyang for its testing of nuclear weapons.
The countries' leaders, Kim Jong Un and Vladimir Putin, in September held a high profile summit in Russia's far east, fanning Western fears Pyongyang might provide Moscow with weapons for its war in Ukraine.
Washington and its key East Asian security allies, Seoul and Tokyo, said they "strongly condemn the provision of military equipment and munitions by the Democratic People's Republic of Korea (DPRK) to the Russian Federation for use against the government and people of Ukraine".
"Such weapons deliveries, several of which we now confirm have been completed, will significantly increase the human toll of Russia's war of aggression."
During his visit to Russia in September, Kim declared bilateral ties with Moscow were his country's "number one priority".
Washington and experts have said Pyongyang was seeking a range of military assistance in return, such as satellite technology and upgrading its Soviet-era military equipment.
The nuclear-armed North has failed twice this year in its bid to put a military spy satellite into orbit.
The Thursday joint statement also claimed Pyongyang was "seeking military assistance to advance its own military capabilities" in "return for its support to Russia."
"We emphasise that arms transfers to or from the DPRK ... would violate multiple UN Security Council resolutions," the statement said.
"Russia itself voted for the UN Security Council resolutions that contain these restrictions," it said, adding the allies were "deeply concerned" about the "potential for any transfer of nuclear- or ballistic missile-related technology" to the North.
In August, Tokyo, Seoul and Washington's leaders met for a summit at Camp David, agreeing to a multi-year plan of regular joint exercises and to share real-time data on North Korea.
Pyongyang's leader Kim has described the burgeoning defence alliance as "the worst actual threat" facing his isolated country.
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