SEOUL--North Korean troops performed artillery drills near the disputed sea boundary with South Korea at the order of leader Kim Jong Un, state media reported Monday, drawing an immediate rebuke from the South amid stalled nuclear talks.

The North’s Korean Central News Agency said that the firing happened when Kim inspected a military unit on Changrin Islet off the west coast.

It was Kim’s first known trip to a front-line military unit since he entered nuclear talks with the United States last year. The nuclear negotiations are largely at standstill now as North Korea steps up pressure on Washington to lift international sanctions and abandon hostile policies on the North.

KCNA said Kim ordered a coastal artillery company to fire before indicating the target for it and that company troops “fully showed their gun firing skills” and “delighted the supreme leader.”

The islet is just north of the inter-Korean maritime boundary, the scene of several bloody naval skirmishes between the rivals in past years. In 2010, North Korea launched an artillery strike on a South Korean island just south of the boundary, killing four people. Earlier that year, North Korea is also accused of torpedoing a South Korean warship operating near the boundary, killing 46 sailors.

South Korea’s Defense Ministry expressed regret over the latest drills, saying they violated last year’s inter-Korean deals aimed at lowering military animosities in border areas.

Spokeswoman Choi Hyun-soo told reporters that North Korea must stop any acts that can increase military tensions in front-line areas and abide by the 2018 tension-reduction agreements.

Relations between the two Koreas improved greatly last year as Kim was engaged in talks with the United States over the fate of his advancing nuclear arsenal. Kim met U.S. President Donald Trump in Singapore in June last year in what was the first North Korea-U.S. summit talks since the end of the 1950-53 Korean War. Kim also met South Korean President Moon Jae-in three times last year.

But the nuclear talks fell apart in February, when Trump rebuffed Kim’s calls for broad sanctions relief in return for dismantling his main nuclear complex, a partial denuclearization step. Inter-Korean relations have subsequently suffered setbacks, with North Korea criticizing South Korea for failing to break away from the United States and restore joint economic projects held by U.N. sanctions.

In recent months, North Korea test-fired a slew of short-range missiles and other weapons, which experts say mainly target South Korea. North Korea has also threatened to dismantle South Korean-built buildings at a now-shuttered joint tourism project in the North.

North Korea hasn’t lifted a self-imposed moratorium on long-range missile and nuclear tests, which Trump has boasted as his major achievement in his North Korea policy. Kim has demanded Trump come up with new, acceptable proposals to salvage the nuclear negotiations by year’s end.