North Korean leader Kim Jong-un watches the test-fire of two short-range ballistic missiles on Thursday, in this undated picture released by North Korea's Central News Agency on July 26. Reuters

North Korea launched two more projectiles believed to be short-range ballistic missiles into the East Sea, Saturday, according to the South Korean military, the latest in a series of saber-rattling moves protesting a joint exercise between Seoul and Washington.

The projectiles were fired at 5:32 a.m. and 5:50 a.m. from the vicinity of the eastern coastal city of Hamhung, South Hamgyong Province, and flew around 400 kilometers at a maximum altitude of 48 km and a top speed of around Mach 6.1, the Joint Chiefs of Staff (JCS) said.

"We see high chances of additional launches, as North Korea is now carrying out summertime drills and the combined exercise between South Korea and the U.S. is underway," the JCS said in a press release.

"Accordingly, we have been closely monitoring the situation, while maintaining the appropriate military readiness," it added.

This is the fifth series of launches since July 25 when Pyongyang fired the first set of two short-range missiles. It came just four days after the North launched two projectiles believed to be the newly developed short-range KN-23 ballistic missiles, also into the East Sea.

Cheong Wa Dae calls NK missile firing 'show of force' against S. Korea-US military drill 2019-08-10 17:11  |  North Korea Trump says he received 'beautiful' letter from NK leader 2019-08-10 09:29  |  North Korea US says it is aware of report of North Korea's missile launch 2019-08-10 09:22  |  North Korea The actions appear to be a show of force and a protest against the combined military exercise between South Korea and the U.S., Cheong Wa Dae said, adding Pyongyang was also testing the capabilities of newly developed short-range ballistic missiles.

Seoul and Washington kicked off their military exercise Monday, with the main part of the computer-based Combined Command Post Training set to begin Sunday for a 10-day run.

Though the allies have said the exercise is defensive in nature, and is meant to test South Korea's operational capabilities for the conditions-based transition of wartime operational control from Washington to Seoul, the North has denounced it as a rehearsal for invasion, and has repeatedly demanded its cancellation and threatened to seek "a new way" rather than engagement.

Sending a letter to U.S. President Donald Trump this week, North Korean leader Kim Jong-un expressed his displeasure with the joint military exercises, Trump said Friday.

"Such a series of launches are feared to heighten military tensions on the Korean Peninsula, and we urge North Korea to stop such activities," Cheong Wa Dae said in a release.

In Washington, a senior U.S. government official said the U.S. was aware of the latest launches and was monitoring the situation closely with its allies. (Yonhap)