The use of North Korean troops by Russia in Ukraine would cross a red line for the US and NATO, according to a US intelligence chief.

The US House Intelligence Chairman Michael Turner made the claim in a letter to President Joe Biden as he warned that the use of North Korean troops was an “alarming and an extreme escalation in the conflict in Ukraine.”

Ukrainian president Vlodymyr Zelensky told reporters at a NATO summit in Brussels that Kim Jong Un is set to send 10,000 soldiers to join Russia's war against Ukraine describing it as "the first step to a world war."

According to the Institute for the Study of War, South Korean National Intelligence Agency (NIS) believes that North Korea has sent 1,500 North Korean special forces to eastern Russia, where they are training before deploying to participate in the conflict.

The NIS has recently shared satellite imagery that it says shows around 700 North Korean soldiers gathered at Russian military facilities in Ussuryisk and Khabarovsk.

NIS said North Korean soldiers were given Russian military uniforms and Russian-made weapons and were issued with fake ID cards of residents of Yakutia and Buryatia, two regions in Siberia.

Ukraine's spy chief Kyrylo Budanov recently claimed that intelligence suggests that the troops will be deployable from the beginning of November.

South Korean Defense Minister Kim Yong-hyun has also stated that it is believed a small number are already fighting around the Donetsk area of Ukraine.

Russia has strengthened its relationship with the secretive state since its invasion of Ukraine, as the two pariah nations seek to work together to counter western sanctions and resource gaps.

It is already known that North Korea has provided Moscow with artillery shells and ammunition to address shortages in Russia’s supply chain.

Putin has visited the North Korean capital Pyongyang and last week, Kim Jong Un greeted Russian President Vladimir Putin on his birthday, calling him his "closest comrade".

It is unknown how effective North Korean troops will prove when working with their Russian counterparts.

Aside from the obvious language barrier, North Korean troops have no combat experience and there is likely to be significant differences in training and standard operating procedures between the two.

That said, throughout the war, as casualty figures have stacked, Russia has continually deployed poorly trained and equipped troops, proving that lack of North Korean experience on the battlefield is unlikely to prove a sticking point.

It has been suggested by some, that the troops might be used to guard sections of the Ukrainian-Russian border, releasing combat-hardened Russians to be deployed elsewhere.

It remains unclear how NATO and other countries will react to the development.

South Korea vowed last week to deliver aid to Ukraine if its northern enemy followed through with plans to deploy troops.

Mr Turner urged Joe Biden in his letter to make it “absolutely clear and unequivocal” that the move is a red line for the US and NATO.