North Korea fired four short-range ballistic missiles off its west coast on Saturday, the South Korean military said, following a series of missile launches in recent days in protest against an extended U.S.-South Korea joint air exercise.

South Korea's Joint Chiefs of Staff said it detected the launches toward the Yellow Sea from Tongrim County in North Pyongan Province between around 11:32 a.m. and 11:59 a.m.

The missiles flew about 130 kilometers at an altitude of about 20 km with a top speed of Mach 5, the JCS said.

The series of missile launches by Pyongyang since the United States and South Korea began their first large joint air exercise in five years on Monday have heightened tensions on the Korean Peninsula amid fears that the North could conduct its seventh nuclear test.

Pyongyang has strongly protested the Vigilant Storm air exercise involving 240 warplanes, firing a barrage of ballistic missiles on Wednesday and Thursday.

The joint exercise was originally planned to run from Monday to Friday, but it was extended given the heightened tensions following the North Korean missile launches.

On Saturday or the last day of the extended drills, two B-1B strategic bombers joined the exercise with four South Korean F-35A stealth fighter jets and 4 U.S. F-16 fighters.

This marked the first time the bomber has been deployed to the peninsula since 2017.

The exercise on Saturday demonstrated the U.S. commitment to deterrence for South Korea and the two allies' defense posture to deal with the North Korea's provocation, the South Korean military said.

South Korea scrambled around 80 fighter jets Friday after about 180 radar tracks of North Korean warplanes were detected near the countries' eastern and western maritime borders.

The country's military plans another exercise from Monday through Thursday.