North Korean leader Kim Jong-un has guided artillery firing drills involving front-line units capable of striking the "enemy's capital," state media reported Friday, in an apparent response to an ongoing joint military exercise between South Korea and the United States.

The drills were conducted Thursday with the aim of enhancing the combat readiness posture and actual war capability of artillery troops, according to the Korean Central News Agency.

"The drill started with the power demonstration firing of the long-range artillery sub-units near the border who have put the enemy's capital in their striking range and fulfilled important military missions for war deterrence," the KCNA said.

The enemy's capital is believed to be referring to Seoul.

Kim's inspection came as Seoul and Washington have been staging the annual Freedom Shield exercise since Monday to strengthen deterrence against North Korea's nuclear and missile threats.

"It is necessary to more vigorously push forward the work for making preparations for regular combat mobilization so that all the artillery sub-units can take the initiative with merciless and rapid strikes at the moment of their entry into an actual war," Kim was quoted as saying by the KCNA.

The North's leader set for important tasks for "rounding off artillery war preparations," the report said, without revealing details.

Kim oversaw military drills by the Korean People's Army for the second straight day Thursday. He called for intensifying "practical actual war drills" during his visit to a military training base in the nation's western region Wednesday.

South Korea's military said it detected the North's live-fire artillery drills toward the Yellow Sea on Thursday in an area around the western port city of Nampho, involving dozens of multiple rocket launchers and self-propelled howitzers, from 11 a.m. to 5 p.m.

The Joint Chiefs of Staff warned that it will retaliate "overwhelmingly" if North Korea undertakes a provocation.

"While maintaining a solid combined defense posture, our military will conduct the ongoing Freedom Shield exercise and combined drills in a substantial manner, and is closely monitoring signs of North Korean provocations and military activity," it said.

Pyongyang has long denounced joint military drills between Seoul and Washington as rehearsals for an invasion and used them as a pretext for provocations. The allies have said their military exercises are defensive in nature.

North Korea's defense ministry warned Monday that South Korea and the US will pay a "dearly price" for what it called their large-scale war drills. (Yonhap)