North Korea fired four ballistic missiles Monday off its east coast, according to South Korea and Japan. The move is an apparent retaliation to joint military drills by the U.S. and South Korea.

Kim Jong Un’s regime launched the missiles from the Dongchang-ri long-range missile site in North Pyongan Province. The projectiles flew about 620 miles before splashing into the Sea of Japan, Seoul’s Joint Chiefs of Staff (JCS) said. It remains unclear what type of missiles were fired.

"We estimate the North fired four ballistic missiles. We are conducting an analysis [with the U.S.] to determine the exact type of missiles fired and other specifications. It will take a while before we can come up with a final analysis [based on U.S. satellite data]," the JCS said, according to South Korea's Yonhap News Agency.

Japanese Chief Cabinet Secretary Yoshihide Suga said three of the four missiles fell into Japan's exclusive economic zone.

American officials told Reuters that there were no indications that Pyongyang tested an intercontinental ballistic missile. The U.S. State Department issued a statement condemning the test-firing.

"The United States strongly condemns the DPRK's [Democratic People’s Republic Korea] ballistic missile launches tonight, which violate U.N. Security Council Resolutions explicitly prohibiting North Korea's launches using ballistic missile technology," the department’s Deputy Spokesman Mark Toner said in a statement.

South Korea’s Acting President and Prime Minister Hwang Kyo-ahn slammed the missile tests and organized a National Security Council meeting after the missile firing, Yonhap reported. He said that Pyongyang’s nuclear and missile provocation are "real and imminent threats for the people's lives and security."

Last Thursday, North Korea warned it would carry out “merciless military counter-actions” if Washington and Seoul provoked the Kim Jong Un-led country. This was in response to the annual joint military drills started last Wednesday by the two countries in the Korean Peninsula.

“Should the U.S. imperialists and the South Korean puppet forces fire even a single shell into the waters where the sovereignty of our Republic is exercised, the KPA will immediately launch its merciless military counter-actions.  They should not forget that our revolutionary armed forces have everything in place and are always on alert,” the General Staff Department of the Korean People's Army said at the time.

Tensions in the Korean Peninsula have flared up over Pyongyang’s nuclear and missile activities. Last month, the reclusive country carried out a missile test triggering international criticism. Pyongyang has been also blamed for violating global nuclear weapons treaties with its ballistic missile tests.