North Korean leader Kim Jong-un / Yonhap

North Korea fired two suspected ballistic missiles eastward Friday, South Korea's military said, after Pyongyang publicly warned earlier in the day of a "stronger and certain" response to the United States' imposition of new sanctions.

The North launched the projectiles from a site in North Pyongan Province, a northwestern region bordering China, the Joint Chiefs of Staff (JCS) said in a text message sent to reporters. It did not elaborate.

The North's latest saber-rattling marks its third show of force this year.

South Korea's military is monitoring related North Korean movements and maintaining a firm readiness posture, the JCS said.

Pyongyang issued the warning earlier in the day, after Washington slapped sanctions on six North Koreans, Wednesday, who it said were involved in the regime's weapons of mass destruction and ballistic missile programs.

"If the U.S. adopts such a confrontational stance, the DPRK will be forced to take stronger and certain reaction to it," a spokesperson of the foreign ministry said in a statement carried by the North's official Korean Central News Agency. DPRK stands for the North's official name, the Democratic People's Republic of Korea.

The North launched what it claimed were hypersonic missiles, Jan. 5 and Tuesday, in an apparent search for new advanced weapons amid a deadlock in denuclearization talks with the United States. (Yonhap)