Russian President Vladimir Putin signed a mutual defence agreement with North Korea's Kim Jong Un, who offered his "full support" on Ukraine.
The pledge of military cooperation on Wednesday was part of a strategic treaty signed during a summit in Pyongyang, where Putin was making his first visit in 24 years.
"The comprehensive partnership agreement signed today provides, among other things, for mutual assistance in the event of aggression against one of the parties to this agreement," Putin said.
Ahead of the visit North Korea appears to have made preparations for a possible military parade in downtown Pyongyang, commercial satellite imagery showed.
In a letter published in North Korea's Rodong Sinmun, a ruling Workers' Party mouthpiece, the Russian president prior to his visit said that the two countries have developed good relations and partnerships over the past 70 years based on equality, mutual respect and trust.
Meanewhile US State Department spokesperson Matthew Miller repeated charges on Monday that North Korea had supplied "dozens of ballistic missiles and over 11,000 containers of munitions to Russia" for use in Ukraine.
He said the United States had seen Putin "get incredibly desperate over the past few months" and look to Iran and North Korea to make up for equipment lost on the battlefield.
Moscow and Pyongyang have denied arms transfers.
Formally known as the Democratic People's Republic of Korea (DPRK), North Korea has been under UN sanctions for its ballistic missile and nuclear programs since 2006, and those measures have been strengthened over the years.
For the past several years the Security Council has been divided over how to deal with Pyongyang. Russia and China say more sanctions will not help and want such measures to be eased. They proposed some sanctions be lifted in December 2019, but have never put their draft resolution to a vote.
In May 2022, the pair vetoed a US-led push to impose more UN sanctions on North Korea over its renewed ballistic missile launches. Russia then vetoed in March this year the renewal of a panel of experts monitoring enforcement of UN sanctions.
China and Russia say joint military drills by the United States and South Korea provoke Pyongyang, while Washington accuses Beijing and Moscow of emboldening North Korea by shielding it from more sanctions.
After North Korea, Putin will visit Vietnam on June 19-20.
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