A suspected North Korean soldier reportedly captured by the Ukrainian military is seen in this undated photo captured from Telegram. Yonhap

An injured North Korean solider died Friday from worsening wounds, a day after being captured by Ukraine, South Korea's spy agency confirmed, citing intelligence from a "friendly country."

"We confirmed through a friendly nation's intelligence organization that a North Korean solider, captured alive on Dec. 26, died a short while ago as (his) wounds worsened," the National Intelligence Service (NIS) said.

The remarks came hours after the agency initially confirmed Ukraine's capture of an injured North Korean soldier, following a news report about Ukrainian special operations troops seizing a North Korean fighter in the first reported instance of its kind.

The NIS confirmed the report as valid.

Ukraine's military-focused news outlet Militarnyi reported Thursday (local time) that Ukraine's forces captured the soldier during operations in Russia's western front-line region of Kursk, without specifying when the incident occurred.

A photo of the purportedly captured North Korean soldier, who appeared to have sustained injuries, circulated on Telegram. Details of the soldier's condition and status remained unknown.

If confirmed, it would mark the first time Ukraine has captured a North Korean soldier engaged in combat, according to Militarnyi.

Some 11,000 North Korean troops have been sent to Russia so far, with signs of preparations for further deployment of troops and military equipment, according to South Korean officials.

Ukrainian President Volodymyr Zelenskyy said earlier this week that North Korea has suffered more than 3,000 casualties in the Kursk region. (Yonhap)