It is unclear what kind of collaboration he was referring to. The ambassador did not confirm allegations that North Korea sent troops to fight the Russian military.

Pyongyang also did not comment on the accusations.

South Korea has long accused the North of supplying weapons to Russia for use in the war against Ukraine, but says the current situation goes beyond the transfer of military materials.

Some South Korean media reported that up to 12,000 North Korean troops were expected to be deployed.

“(This) seriously threatens not only South Korea but also the international community,” Kim said Monday.

Moscow and Pyongyang stepped up cooperation after their leaders Vladimir Putin and Kim Jong Un signed a security pact in June, promising their countries would help each other in the event of “aggression” against either country.

Last week, Putin introduced a bill to ratify the pact.

Pyongyang’s deployment of troops to fight Russia “would mark a significant escalation” of the conflict, NATO Secretary General Mark Rutte said on Monday.

The United States and Japan also condemned the deepening military ties between North Korea and Russia.

Some defense experts told BBC Korean that North Korean intervention could complicate the war.

“North Korea’s participation could open the door to broader international involvement in the conflict, potentially involving more countries,” said Moon Seong-mok of the Korea Institute for National Strategy.

“The international community will likely increase sanctions and pressure on both Russia and North Korea, but it remains to be seen whether North Korea’s participation will truly benefit either country,” Dr Moon said.

But others believe Russian military units will have difficulty incorporating North Korean troops into their front lines.

Besides the language barrier, they say, the North Korean army has no recent combat experience.

Valery Ryabykh, editor of the Ukrainian publication Defense Express, said that North Korean soldiers could be tasked with guarding sections of the Russian-Ukrainian border, which would free up Russian units to fight elsewhere.

“I would rule out the possibility of these units immediately appearing on the front line,” he said.

Additional reporting by Sunmi Han, Jake Kwon and Hosu Lee in Seoul