The U.S.-led U.N. Command(UNC) has reportedly concluded that both South and North Korea violated the armistice last month by sending drones across the border into each other's territory.

According to a military source on Wednesday, the UNC recently reached the conclusion after a special investigative team reviewed the North's drone incursion on December 26.

The UNC has reportedly not yet notified the South Korean government of its conclusion. 

The military source said that the UNC often wraps up its investigations and reports the results to the UN, without notifying South Korea. 

On December 26, North Korea sent five unmanned aerial vehicles across the Military Demarcation Line into South Korean airspace, which led the South to send three drones into the North in a "corresponding" step to conduct reconnaissance and take photos of military facilities. 

Earlier this month, the South Korean defense ministry defended its decision to send drones into North Korean territory as exercising its right to self-defense, stressing that it is a legitimate right guaranteed under the UN Charter.