Officials in Ukraine and South Korea have accused North Korea of dispatching troops, estimated to number in the thousands perhaps, to aid Russia in the war in Ukraine.
Both Moscow and Pyongyang have denied the claims and US officials have said they are still investigating the reports.
What do we know?
South Korea's foreign ministry summoned the Russian ambassador in Seoul on Monday to protest over what it has called the sending of North Korean troops to Russia for deployment in Ukraine.
South Korea's first vice foreign minister Kim Hong-kyun said the participation of North Korean troops in the war in Ukraine violated UN resolutions and the UN charter and posed serious threats to the security of South Korea and beyond.
"We condemn North Korea's illegal military cooperation, including its dispatch of troops to Russia, in the strongest terms," the ministry quoted Kim as saying.
The Kremlin declined to directly answer a query on whether North Korean troops were going to fight in Ukraine, but spokesman Dmitry Peskov said Moscow's cooperation with Pyongyang was not directed against third countries.
Last week, Ukraine's President Volodymyr Zelenskyy
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On Friday, South Korea's National Intelligence Service (NIS) said the North had shipped 1,500 special forces troops to Russia's Far East for training and acclimatising at military bases there, probably to be deployed for combat in the war.
The South also said it had used AI facial recognition technology to identify a delegation of dozens of North Korean officers visiting frontline areas in Ukraine, to give guidance on North Korea-made KN-23 ballistic missiles fired at targets there.
Vessels of Russia's Pacific Fleet were detected moving the North Korean special forces troops to Vladivostok from 8 to 13 October and are expected to resume that effort soon, the NIS said.
The troops have been supplied with Russian military uniforms, weapons and false identification documents ahead of being deployed for combat, it said.
On Saturday US Defence Secretary Loyd Austin said he could not confirm reports that North Korea had sent troops to Russia ahead of what could be a deployment to the war in Ukraine, but added that such a move would be concerning, if true.
Military analysts say North Korean troops would not be a game changer but could help prolong the conflict. Source: AP / Ng Han Guan
Why North Korea?
Russian President Vladimir Putin has said a treaty he signed with Kim Jong Un during a visit to Pyongyang in June included a mutual assistance clause for each side to help the other repel external aggression.
Asked if this meant that Russia could be drawn into backing Pyongyang in a conflict on the Korean peninsula or that North Korea could side with Russia in a conflict with the West, Russia's Peskov said the pact "implies truly strategic deep cooperation in all areas, including security".
Analysts say North Korea could benefit from providing weapons and troops by gaining experience and insight from operating on a battlefield.
Pyongyang, heavily sanctioned for its nuclear weapons programme, also appears to be gaining major imports of oil and other products from Russia, according to foreign intelligence agencies and commercial satellite imagery examined by analysts.
A report by a think tank affiliated with South Korea's NIS estimated that the cash-strapped North earned about $811 million last year from arms sales to Russia.
