North Korea has started sending troops to fight with Russia in Ukraine, South Korea's spy agency has said as Seoul warned of a "grave security threat".

The allegation comes a day after Ukrainian President Volodymyr Zelensky said he believed 10,000 North Korean soldiers could join the war, based on intelligence information.

South Korean President Yoon Suk Yeol called for a security meeting on Friday and said the international community must respond with "all available means".

According to the spy agency, 1,500 troops have already arrived in Russia with anonymous sources telling South Korean media the final figure could be closer to 12,000.

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