North Korean leader Kim Jong-Un has overseen a live-fire artillery drill near the disputed sea border with South Korea, state media said Thursday as the South's prime minister visited the flashpoint area.

The Yellow Sea border has witnessed bloody North-South clashes in the past and, with military tensions at their highest level for years, is seen as the prime location for another confrontation.

It was not immediately clear when Kim supervised the live-fire drill. He made an inspection tour of the same artillery units last week that was widely covered by state TV and the Korean Central News Agency (KCNA).

During the latest drill, "shells intensively hit the imaginary targets of the enemy while the roar of the artillery pieces rocked heaven and earth", KCNA said.

"A modern war is an artillery war," Kim was quoted as saying.

North Korea has threatened to unleash a second Korean War -- backed by nuclear weapons -- in response to UN sanctions imposed after its third atomic test in February and joint South Korea-US military manoeuvres.

So far, the land and sea border dividing the two Koreas has remained calm, if tense, and the South has dismissed the North's threats as a crude attempt to put "psychological pressure" on Seoul.

Nevertheless, South Korean Prime Minister Chung Hong-Won on Thursday visited one of the frontline islands near the disputed sea border, and urged troops to prepare for any aggression from the North.

"As a strong buttress protecting the lives and property of the people, I ask for your full readiness ... to respond properly to any North Korean provocations," Chun was quoted as saying by the Yonhap news agency.

The island visited by the premier, Yeonpyeong, was the target of North Korean shelling in November 2010 that killed four people, including two civilians.

He asked local residents -- many of whom have reportedly taken to sleeping fully clothed -- to stay calm and trust the government's commitment to their protection.