The exercise took place on Changrin Islet in the Yellow Sea (West Sea) about 15 km (9.3 miles) north of the maritime border, during North Korean leader Kim Jong-un's visit to a military detachment located there, a South Korean Defence Ministry spokesperson was quoted as saying by Efe news.
The Defence Ministry expressed its concern over the act, claiming it violated the agreement signed by Kim and South Korean President Moon Jae-in at the summit held in Pyongyang in September last year.
The authorities at Seoul however did not specify when the military exercise was carried out. North Korean state media reported it Monday.
Changrin, located about 130 km northwest of the South Korean port city of Incheon, is very close to the so-called Northern Limit Line drawn by Seoul and Washington at the end of the Korean War (1950-1953), which Pyongyang does not accept.
North Korean state news agency KCNA said that Kim ordered prominent soldiers on the island to "set up a well-knit system" to ensure "full readiness for carrying out a combat mission any moment".
"We call on North Korea to immediately stop all military actions in border areas, which are feared to heighten military tensions, and to fully comply with the pact," Seoul's Defence Ministry spokesperson Choi Hyun-soo said at a press conference, according to Yonhap news agency.
Following the rapprochement between the two Korean nations in 2018, the lack of progress in denuclearisation talks with the US especially regarding lifting of sanctions and resumption of cooperation projects between the two countries has led Pyongyang to return to a more hostile attitude towards Washington and its southern neighbour.
( With inputs from IANS )