North Korea says leader Kim Jong-un supervised Friday’s test-firing of an unspecified “new weapon”, seen as an attempt to pressure Washington and Seoul over slow nuclear negotiations and their joint military exercises.

Pyongyang’s Korean Central News Agency (KCNA) said on Saturday that Kim expressed “great satisfaction” over his military’s “mysterious and amazing success rates” in recent testing activity and vowed to build up “invincible military capabilities no one dare provoke.” The report did not mention any specific comment on the United States or South Korea.

The launches were North Korea’s sixth round of tests since late July that revealed developments of a new rocket artillery system and two separate short-range mobile ballistic missile systems, which experts say would expand its ability to strike targets throughout South Korea, including US bases there.

KCNA did not describe what Friday’s weapons were or how they performed, but it said that the tests were successful and strengthened the military’s confidence in the reliability of the system.

Pyongyang’s official Rodong Sinmun newspaper published photos that showed what appeared to be a missile soaring from a launcher installed on a vehicle and striking what appeared to be a coastal target. Kim is seen jubilantly raising his fist while celebrating with military officials.

“[Kim] said everyone should remember that it is the [ruling] party’s core plan and unwavering determination to build a powerful force strong enough to discourage any forces from daring to provoke us and to leave any opponent defenceless against our Juche weapons of absolute power even in situations of physical clashes,” the agency said, referring to the North’s national ideology of self-reliance.

South Korea’s military said earlier that two projectiles launched from the North’s eastern coast flew about 230km (143 miles) before landing in waters between the Korean Peninsula and Japan. The US and South Korean militaries were analysing the launches but didn’t immediately say whether the weapons were ballistic missiles or rocket artillery.

Experts say President Donald Trump’s repeated downplaying of the North’s launches allowed the country more room to intensify its testing activity and advance its short-range weaponry while it seeks to build leverage ahead of nuclear negotiations with Washington, which could resume after the end of ongoing allied military drills later this month.

North Korea has ignored South Korean calls for dialogue recently and is seen as trying to force Seoul to make stronger efforts to coax major concessions from Washington on its behalf.