The hackers are known to conduct spearphishing campaigns posing as real journalists, academics, or other individuals with credible links to North Korean policy circles, Seoul's foreign ministry says.

In a rare admission of a North Korean setback, reports that the Chollima-1 rocket, carrying a military reconnaissance satellite known as 'Malligyong-1', crashed into the sea after an accident.

The flight was supposed to put North Korea's first spy satellite in orbit.

The nuclear-armed North says it has completed its first military spy satellite and leader Kim Jong Un has approved final preparations for the launch.

Analysts say the military satellite is part of nuclear-armed North Korea's efforts to advance surveillance technology, including drones, to improve its ability to strike targets in the event of a conflict.

Howitzers pounded into a mountainside in Pocheon near the fortified border with North Korea, while battle tanks manoeuvred and fired their guns at targets, sending smoke, dust and shockwaves across the valley.

US and South Korean forces have been staging an intensified series of annual springtime exercises since March, including air and sea drills involving a US aircraft carrier and heavy bombers.

With North Korea launching ballistic missiles at an unprecedented pace in the past year, Japan, South Korea and the US in November agreed to speed up information-sharing.

The statement suggests North Korea's cycle of military shows of force and weapons development will continue.

They also issue the Washington Declaration, bolstering the US nuclear umbrella over South Korea, which is increasingly nervous about the sabre-rattling in the north.

In the most severe action ever taken by US authorities against a company for breaking North Korea sanctions, BAT's Singapore subsidiary also agreed to plead guilty to felony charges of bank fraud and sanctions-breaking.

Kim's visit to the space agency comes just days after North Korea tested a new solid-fuel intercontinental ballistic missile, a development experts say would facilitate missile launches with little warning.