North Korean leader Kim Jong Un supervised artillery firing drills aimed at boosting combat readiness, state media reported Friday, days after his country vowed to take corresponding military steps against the ongoing South Korean-US military training that it regards as an invasion rehearsal.

Thursday's drills involved frontline artillery units, whose weapons place Seoul, the South Korean capital, in their striking range, the North's official Korean Central News Agency said.

Kim said artillery units must “take the initiative with merciless and rapid strikes at the moment of their entry into an actual war,” KCNA said.

South Korea's Joint Chiefs of Staff said later Friday that it had detected North Korea firing artillery into the waters off its west coast the previous day. It said South Korea will make a stern and overwhelming response in the event of provocations by North Korea.

North Korea's forward-deployed long-range artillery guns pose a serious security threat to Seoul, a city with 10 million people which is about 40 to 50 kilometres (25 to 30 miles) from the border with North Korea.

North Korea's Defence Ministry said Tuesday it would conduct unspecified "responsible military activities” in response to the annual South Korea-US military drills that began on Monday. Kim visited a western operational training ground on Wednesday and called for stronger war fighting capabilities.

The 11-day South Korean-US drills involve a computer-simulated command post training and 48 kinds of field exercises, twice the number conducted last year.

North Korea views South Korea-US military exercises as a major security threat, calling them a preparation to launch attacks on the North. Seoul and Washington officials have said their drills are defensive in nature.

North Korea has sharply accelerated its missile testing activities since 2022 in part of efforts to develop more powerful nuclear-capable weapons targeting the US mainland and South Korea. The South Korean and US militaries have expanded their drills in response.

Experts say North Korea likely aims to use a modernised arsenal to win sanctions relief from the United States when diplomacy restarts. They say North Korea could increase its weapons tests and dial up warlike rhetoric this year as the United States and South Korea hold major elections.