President Yoon Suk Yeol speaks during his phone call with European Commission President Ursula von der Leyen at the presidential office in Seoul, Monday, in this photo provided by his office. Yonhap

President Yoon Suk Yeol said Monday that North Korea's troop deployment in Russia's war with Ukraine could take place "sooner than expected," calling the latest development "serious."

Yoon shared information on North Korea's troop movement during his phone call with European Commission President Ursula von der Leyen, after Ukrainian President Volodymyr Zelenskyy said North Korean troops could join the battlefield against Ukraine "in a matter of days."

"This unlawful military cooperation between Russia and North Korea fundamentally undermines the rules-based international order and poses a threat to peace on the Korean Peninsula and globally," Yoon said.

Yoon called for the immediate withdrawal of North Korean troops and the cessation of military cooperation between Russia and North Korea, vowing to explore "practical response measures" with the European Union and its member countries.

"We will take phased measures depending on the developments in Russia-North Korea military cooperation," he said.

Last week, Yoon said Seoul could consider providing arms to Kyiv, in a possible shift from its policy of only sending humanitarian aid.

South Korea and the United States have said North Korea sent around 3,000 troops to eastern Russia, with Seoul's spy agency expecting about 10,000 soldiers to be deployed by December. (Yonhap)