North Korean leader Kim Jong-un inspects a front-line military unit on the border islet of Changrin, according to the Korean Central News Agency, Monday. Yonhap

North Korean leader Kim Jong-un has inspected a front-line military unit on an island just north of the western sea border with South Korea and ordered firing drills, Pyongyang's official news agency said Monday.

South Korea expressed regret over the artillery fire from the border islet of Changrin, saying that it represents a violation of a military agreement between the two Koreas to avoid tension-heightening activity along their border.

The islet lies just north of the Northern Limit Line (NLL), a de facto maritime border with South Korea, which was the site of bloody naval skirmishes between the two Koreas in the past.

The Korean Central News Agency said that Kim ordered soldiers "to set up a well-knit system" to ensure "full readiness for carrying out a combat mission any moment." He also "indicated the target for Gun 2 of the coastal artillery company on combat duty and gave an order to fire."

KCNA did not provide further details.

During the inspection, Kim ordered soldiers "to set up a well-knit system" to ensure "full readiness for carrying out a combat mission any moment." Yonhap

South Korea protested the firing drills as a violation of a military agreement the two Koreas signed in September last year to avoid activities that increase tensions near the border.

"The artillery firing drills that North Korea mentioned is in violation of the Sept. 19 military agreement that the military authorities of the two countries agreed and have fully implemented," ministry spokesperson Choi Hyun-soo told a regular briefing.

"We call on North Korea to immediately stop all military actions in border areas that are feared to heighten military tensions, and to fully comply with the pact," she added.

In a separate report, Pyongyang's state media reported that leader Kim also visited a "women's company under Unit 5492 of the Korean People's Army" stationed "at the remote seaside on the southwestern front, "apparently referring to the Changrin islet.

It is the first time that Kim has toured the islet since taking office in late 2011. KCNA said that Kim's late father, former leader Kim Jong-il, visited there decades ago.

The latest "field guidance" was his third reported visit to military installations this month. He attended an air show on Nov. 16 and supervised an airborne landing training two days later, underlining the need to improve war readiness.

It came amid North Korea's ramped-up demand for Washington to drop its "hostile policy" toward its regime and come up with an acceptable proposal before its self-imposed end-of-year deadline.

Nuclear negotiations have been stalled since the second summit between Kim and U.S. President Donald Trump broke down in February as they failed to find common ground over how to match Pyongyang's denuclearization steps and Washington's sanctions relief.

The two counties held working-level talks last month for the first time since the summit breakdown but failed to produce any tangible progress.

Observers say that Kim's trip to the border islet might be intended to put pressure not just on the U.S. in making more concessions in future talks but also on South Korea to play a role in the process. (Yonhap)