Rutte said he spoke to South Korean President Yoon Suk Yeol about the Alliance's "close partnership with Seoul, defence industrial cooperation, and the interconnected security of the Euro-Atlantic and Indo-Pacific".

"North Korea sending troops to fight alongside Russia in Ukraine would mark a significant escalation," he added in a post on X.

Seoul's spy agency said on Friday that North Korea had decided to send a "large-scale" troop deployment to support Moscow's war in Ukraine, with 1,500 special forces already in Russia's Far East and undergoing training.

The agency estimated the North could send around 12,000 soldiers in total.

Rutte had said on Friday that NATO could not confirm South Korean intelligence "at this moment" but added: "This, of course, might change."

Pyongyang and Moscow have been allies since North Korea's founding after World War II.

They have drawn even closer since Russia's 2022 invasion of Ukraine, with Seoul and Washington claiming that Kim Jong Un has been sending weapons for use in Ukraine.