North Korea has tested a “newly developed ultramodern tactical weapon” in an apparent bid to apply pressure on the United States and South Korea.

State media reported leader Kim Jong Un had observed the test but the exact nature of the device was not revealed.

The state-run KCNA news agency said the test was successful and the weapon could protect North Korea like a “steel wall”, adding that Kim had observed “the power of the tactical weapon”.

“This result today is a justification of the party’s policy focussed on defences science and technology, another display of our rapidly-growing defences capabilities to the whole region, and a groundbreaking change in strengthening our military’s combat capabilities,” the North Korean leader said.

In a possibly conciliatory gesture, however, the country also announced on Friday it was releasing an American citizen detained since October after “illegally” entering North Korea from China.

It did not appear to be a test of a nuclear device or a long-range missile with the potential to target the US and could well be a bluff – the secretive dictatorship has a history of using fake and photoshopped weapons to project military might.

The only picture released by state media showed Kim standing on a beach surrounded by officials in military uniforms, but no weapons were visible.

The North’s first mention of new weapons development in months threatens to sour the political atmosphere, as negotiations between Pyongyang and the United States appear to have stalled.

The North has not publicly tested any weapons since November 2017, but in recent days Pyongyang reportedly expressed anger at US-led international sanctions and ongoing small-scale military drills between South Korea and the United States.

Earlier this month, North Korea’s Foreign Ministry warned it could bring back its policy of bolstering its nuclear arsenal if it does not receive sanctions relief.

“It’s North Korea-style coercive diplomacy. North Korea is saying ‘If you don’t listen to us, you will face political burdens’,” said analyst Shin Beomchul of Seoul’s Asan Institute for Policy Studies.

Diplomacy has stalled since a summit between Kim and US President Donald Trump in June, with Washington pushing for more action on nuclear disarmament and the North insisting that the US first approve a peace declaration formally ending the Korean War and lift sanctions.