Russia's plans to bolster its invasion of Ukraine with North Korean troops has sparked warnings from Seoul, which says it could supply weapons to Kyiv for the first time.
North Korea has long provided ammunition and artillery shells to Russia, as it sells its stockpile of Soviet-era weapons to replenish dwindling frontline supplies while bolstering its own fledgling economy.
But, having sent hundreds of thousands of soldiers to their death, compounded by so-called "meat grinder" tactics, Russia is now desperate for another resource: manpower.
"We know about 10,000 soldiers of North Korea that they are preparing to send to fight against us," Ukrainian President Volodymyr Zelenkyy said last week.
"That's really, I think, a big problem," he added.
Ukraine is reporting some 11,000 North Korean troops had already been sent and will be ready to fight by November.
South Korea has provided different figures, stating 1,500 special forces troops were already in Russia, and a total of 12,000 soldiers would end up supporting the invasion.
On Monday, Seoul summoned Russia's ambassador to relay its "grave concerns regarding North Korea's recent dispatch of troops".
By Tuesday, top South Korean officials had convened for an emergency meeting.
Later, a spokesperson told reporters: "We would consider supplying weapons for defensive purposes as part of the step-by-step scenarios, and if it seems they are going too far, we might also consider offensive use."
Neither Russia nor North Korea has acknowledged the deployment.
The revelations pose a threat not only to Ukraine, which is again losing ground to a relentless Russian campaign, but also South Korea, which fears North Korean troops will gain real battle experience and even military secrets and know-how from the Russians.
So far, South Korea has only provided non-lethal assistance to Ukraine, but that could change depending on how Russia utilises the North Korean troops.
"There are so many upsides that North Korea can accrue by sending troops to the battlefield between Ukraine and Russia," said Dr Bong Youngshik, from Yonsei University.
"North Korean troops will gain precious experience of the real battleground experience, and real opportunity to test their own weapon systems, which will help further advancement of those weapons systems, which can be used against South Korea in near future."
While neither Ukraine nor Russia release the official numbers of their dead, observers estimate casualties on both sides to be in the hundreds of thousands.
Russia's 'very wasteful way of fighting'
Exactly where the soldiers are remains to be seen.
People in Russian military groups on the encrypted messaging service Telegram have been sharing videos, claiming they show North Korean troops in Russia's Primorsky Krai region.
Primorsky Krai shares a border roughly 17 kilometres long with North Korea.
In one video, filmed behind barbed wire, people in uniform can be seen streaming into a compound.
In another video, shared on the social media platform X by Ukraine's Centre for Strategic Communications, men could be seen gathering uniforms inside a building.
The government agency claimed they were North Korean soldiers collecting their new combat gear in Samara.
Russia has not confirmed the reports but stated it had the sovereign right to cooperate with its neighbour, North Korea.
NATO chief Mark Rutte, in a social media post, said North Korean troops fighting in Ukraine "would mark a significant escalation" in the conflict.
Dr Jenny Mathers, a senior lecturer in international politics at Aberystwyth University, said it was not surprising Russia would be keen to use North Korean soldiers in its invasion.
"A war Russia thought it would win easily in a matter of days or weeks has dragged on for more than two-and-a-half years and has almost certainly cost them hundreds of thousands of soldiers," she said.
"Russia is struggling to recruit enough men to fill the ranks to fight the war in the way that Russia has chosen to fight it, which is a very wasteful way of fighting it in terms of human life."
In a bid to attract manpower, Russia's increasing payments for civilians that sign up, while offering convicted criminals the chance to avoid prison if they also go to war.
"They're really trying almost everything they can to get enough men without actually having to go to another formal round of mobilisation, because that will be really unpopular," Dr Mathers said.
[Map]But there are questions about how and where the North Korean troops would be utilised.
Dr Bong expects the bulk of North Korean support to be on the back-end, plugging manpower gaps created due to Russia's conscription, while also helping Russians operate North Korean-supplied weapons systems.
Having thousands of troops on the front line would also be seen as crossing a red line for South Korea, which may prompt it to send sophisticated military hardware to Ukraine.
"Russia is going to have far more to lose compared to what it may gain," Dr Bong said.
"South Korea's interest is to keep its strategic cards, not just one strategic card in retaliation, but multiple strategic cards, close to its chest."
Lure of tech secrets for Pyongyang
In June, North Korean leader Kim Jong Un hosted Russian President Vladimir Putin in Pyongyang, where they signed a new military alliance.
Last week, Putin introduced draft law to parliament that states that in the event one of the countries "is subjected to an armed attack" and "thus finds itself in a state of war", the other "shall immediately provide military and other assistance with all means at its disposal".
North Korea commands one of the world's biggest armies, with around 1.3 million active personnel.
It also has about 50 ready-to-fire nuclear warheads, according to the Stockholm International Peace Research Institute.
But, having been cut off from the world for decades, North Korea lacks technological sophistication, and is desperate to make its ballistic missile program more accurate and effective.
This is where Russia can help, which has an advanced space program, and the world's largest nuclear stockpile.
How much Russia will help North Korea is a great unknown, but some analysts are worried.
"We're talking tech required for producing deadly weaponry that would pose a threat to the entire region," Alexander?Kovalenko, a military-political observer who's part of the Information Resistance group Mr Kovalenko wrote in a post on Telegram.
The Information Resistance group is part of the Centre for Military and Political Studies, a non-government organisation based in Kyiv.
Dr Mathers said access to military technology was paramount to North Korea's regime.
"The North Korean leadership has shown itself to be quite unpredictable and very worrisome in terms of what they might do with any more advanced technology," she said.
"Anything that strengthens North Korea is going to be a matter of concern to democratic countries like Australia, the United Kingdom and Untied States, not to mention South Korea, and their allies."
