SEOUL, South Korea (AFP) — South Korea's military scrambled stealth jets on Friday after detecting the mobilization of 180 North Korean warplanes, Seoul said as it conducted large-scale joint air drills with the United States which have infuriated Pyongyang.

"Our military detected around 180 North Korean warplanes" mobilized in Pyongyang's airspace, Seoul's Joint Chiefs of Staff said, adding that Seoul "scrambled 80 fighter jets including F-35As" while jets involved in the joint drills were also "maintaining readiness."

Shortly after South Korea announced the decision to extend the joint drills on Thursday, Pyongyang launched three more short-range ballistic missiles, calling the move "a very dangerous and wrong choice."

Hours later, the North fired 80 artillery rounds that landed in a maritime "buffer zone," Seoul's military said.

The barrage was a "clear violation" of the 2018 agreement that established the buffer zone in a bid to reduce tensions between the two sides, Seoul's Joints Chiefs of Staff said.

The artillery fire came after Pyongyang fired about 30 missiles Wednesday and Thursday, including an intercontinental ballistic missile and one that landed near South Korea's territorial waters for the first time since the end of the Korean War in 1953.

Seoul and Washington extended their largest-ever joint air drills through Saturday in response to the North's flurry of projectiles.