That is what Ukrainian President Volodymyr Zelenskyy has warned during a trip to NATO headquarters in Brussels, calling any North Korean involvement "the first step to a world war".

He also revealed that "tactical personnel" as well as military officers from the totalitarian state were already on the ground in Russian-occupied parts of Ukraine.

"From our intelligence, we've got information that North Korea sent tactical personnel and officers to Ukraine on temporary occupied territories, and they are preparing on their land, 10,000 soldiers, but they didn't move them already to Ukraine or to Russia," Mr Zelenskyy said at a news conference with Mark Rutte, the new secretary general of NATO.

The intelligence has yet to be confirmed by Western officials but the UK, the US and other allies are understood to be monitoring the situation closely.

The deployment of North Korean soldiers would be a significant step up in support of Moscow from Pyongyang, which has already provided large amounts of ammunition, including artillery shells and ballistic missiles.

The move would also be a clear indication of the toll that devastating casualty rates among Russian forces in Ukraine have had on Russian President Vladimir Putin's ability to generate sufficient numbers of new Russian recruits to push forward.

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Western officials claim that the daily casualty rate in September for Russian military personnel - killed and injured in Ukraine - is 1,271.

That level of attrition means even thousands of North Korean troops equate to "not many days of casualties that is sort of plugging the gap", a Western official told a briefing of journalists.

There are also questions about how the North Koreans would integrate into a Russian chain of command.

"At a practical level how does command and control work?" the official said, referring to how Russian commanders on the ground in Ukraine would be able to tell any North Korean soldiers what to do.

"Do they [the North Koreans and the Russians] have radios that talk to each other? Can they speak in a common language? How do you integrate them into forces?" the official said.

Another issue that must surely be on the mind of North Korea's reclusive leader, Kim Jong Un, is the potential for any soldiers he sends away from his isolated nation to defect and refuse to return.

"I think that's a really interesting point," the Western official said.

"It could quite fundamentally undermine - not necessarily the whole thing - but certainly the notion. And if there are significant numbers that is a really interesting dynamic for us."