North Korea fired four short-range ballistic missiles into the sea on Saturday, according to South Korea’s Defense Ministry. The rockets are said to have flown some 130 kilometers, reaching an altitude of 20 kilometers.

The launches bring the total number of ballistic missile tests conducted by Pyongyang since Wednesday to 30. The heightened activities come as the US and South Korea continue their ‘Vigilant Storm’ aerial drills, which kicked off on Monday. Aside from firing ballistic missiles, North Korea has held live-fire artillery exercises, along with other military exercises.

Originally scheduled to conclude on Friday, the war games were extended for one additional day, with Pyongyang denouncing the decision as an “irrevocable and awful mistake.” On Friday, Pyongyang scrambled about 180 fighter jets north of the Military Demarcation Line which separates the two Koreas, in a new show of force.

This year’s ‘Vigilant Storm’ drills involve some 240 military aircraft and are the largest so far, according to the US Air Force. The joint US-South Korean exercise was first held in 2015 under the name ‘Vigilant Ace’ and was set to be conducted annually. The drills were put on hold in 2018 when Seoul tried to mend ties with Pyongyang.