North Korea on Saturday fired two short-range ballistic missiles into the Sea of Japan, the latest in a series of sabre-rattling moves against the joint exercise between Seoul and Washington, South Korea’s military said.

The missiles, which were fired from near the northeastern city of Hamhung, flew 400 km at a peak altitude of 48 km, according to the South Korean military.

The South’s presidential Blue House said the weapons were believed to be short-range ballistic missiles, raising the prospect that they were similar to recent tests of its KN-23 missile, which appears to be built to avoid detection.

“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 US is underway,” the Joint Chiefs of Staff said in a statement.

“We have been closely monitoring the situation while maintaining staunch military readiness.” the statement further said.

US President Donald Trump has received another “beautiful letter” from Pyongyang hours after the two projectiles were launched.

The latest launch came just four days after the North fired two projectiles believed to be the newly developed short-range ballistic missiles known as KN-23 into the East Sea.

“The US was aware of the latest firing and is monitoring the situation closely with the allies”, a senior government official told Yonhap News Agency.

Earlier this month, North Korea had threatened to carry out more weapons after its fourth pair of projectiles in less than two weeks following the start of joint exercises between the US and the South.

Although the UNSC has banned North Korea from ballistic missile launches resolutions, and despite a meeting between North Korean leader Kim Jong Un and US President Donald Trump last month, there has been no change in their stance.

Last week, US President Donald Trump had underlined his personal support for North Korean leader Kim Jong-un, even as he admitted that Pyongyang’s recent missile tests “may” violate the UN resolution.

Last month, North Korean leader Kim Jong had supervised the demonstrations of a new type of tactical guided weapon as a warning to South Korea to stop importing high-tech weapons and conducting joint military exercises with the United States, according to reports.

He also accused South Koreans of “double-dealing” for saying they support peace but simultaneously importing new weapons and conducting military drills.

North Korea had earlier warned that the upcoming US-South Korea joint military exercises could affect the resumption of the talks.