The United States and South Korea began major air exercises on Monday, involving 130 warplanes from both countries to simulate 24-hour wartime operations.
The annual drills, called Vigilant Defense, will run until Friday, featuring variants of the F-35 stealth fighter from both the United States and South Korea, among other aircraft, South Korea's Air Force said.
The drills are aimed at improving interoperability between the two militaries by performing major air missions such as air-to-surface live fire drills, defensive counter air operation and other emergency training.
"We will maintain the best combat readiness to immediately respond to and strongly punish any provocation by the enemy through an intense training simulating an actual situation," South Korea's military said in a statement.
The drills come as North Korea has been stepping up military cooperation with Russia in what the United States and its Asian allies have condemned as Pyongyang's efforts to advance its military capabilities in return for arms support to Moscow.
North Korean Foreign Minister Choe Son Hui said on Saturday it was the "steadfast will" of Pyongyang to expand ties with Russia, adding that their relations will act as a "powerful strategic" element if security in the region is endangered.
The U.S. special representative for North Korea, Sung Kim, held a videoconference with his Chinese counterpart Liu Xiaoming Monday. In a statement the State Department said the two discussed North Korea’s “increasingly destabilizing and escalatory behavior,” and its military cooperation with Russia.
North Korea has long condemned joint drills between the United States and South Korea as a rehearsal for invasion and proof of hostile policies by Washington and Seoul.