This June 21, 2017 file photo shows a North Korean unmanned aerial vehicle (UAV) displayed in a briefing room at the Ministry of National Defense. Korea Times file

The United States reaffirmed its commitment to the defense of South Korea on Monday after North Korean drones crossed the inter-Korean border.

The South Korean military said a group of five North Korean unmanned aerial vehicles had crossed the border on Monday (Seoul time), and that at least one of them was spotted flying over northern Seoul.

A spokesperson for the White House National Security Council also said the U.S. was consulting closely with South Korean allies to find the nature of North Korea's latest provocation.

"We are aware of the reported DPRK drone flights across the military demarcation line, and we are consulting closely with the ROK about the nature of this incursion," the spokesperson told Yonhap News Agency in an email, referring to North Korea by its official name, the Democratic People's Republic of Korea.

North Korean drones trespass into South Korean airspace 2022-12-26 20:32  |  North Korea
"We recognize the need of the ROK to protect its territorial integrity. The U.S. commitment to the defense of the Republic of Korea remains ironclad," the official added.

ROK stands for the Republic of Korea, South Korea's official name.

The South Korean military said they fired some 100 shots while trying to limit possible damage to civilians, but failed to shoot the drones down.

"This is a clear act of provocation by North Korea that encroached upon our territorial air," an official from South Korea's Joint Chiefs of Staff said earlier. "Our military will respond thoroughly and sternly to such a North Korean provocation going forward."

The drone infiltration follows a record number of North Korean ballistic missile tests this year.

Pyongyang fired more than 60 ballistic missiles this year, including eight intercontinental ballistic missiles, far exceeding its previous annual record of 25. (Yonhap)