North Korean leader Kim Jong-un inspected troops on two islands near the border with South Korea in the Yellow Sea, to check the readiness of his military amid escalated tensions with the US and Seoul.

Kim went to inspect the detachments on the islets of Jangjae and Mu and was "greatly satisfied with the combat readiness" of the soldiers, the North Korean state agency KCNA reported on Friday.

After having looked at South Korea's Yeonpyeong Island from the observation post of the defence detachment on Jang Jae Islet and receiving a report on the recent reinforcement on this island, the North Korean leader "acquainted and examined the plan for fire strike" on South Korea's newly organised forces, Efe news reported.

He stressed the need for the artillery group to "keep highly alert to break the backbone of the enemy once ordered", KCNA said.

Kim then moved to Mu Islet, a military base that in November 2010 fired about 170 rounds of artillery against Yeonpyeong, killing two South Korean civilians and two soldiers.

The incident was North Korea's first direct attack on South Korea since the Korean War (1950-1953).

"The duty of the two defense detachments are very important," the North Korean leader said while giving instructions for "rounding off" the combat preparation.

On both islets, Kim had a photo session with the service members as well as the families of officers and visited the newly-built facilities for converting seawater into fresh water to supply the troops.