North Korea's artillery attack on South Korea was likely related to Pyongyang's succession plans for the regime's leader-in-waiting, the US military's top officer said Wednesday.

"This is also tied, we think, to the succession of this young 27-year-old who's going to take over at some point in the future," Admiral Mike Mullen said in an interview, referring to Kim Jong-Un, the youngest son of the current leader.

"It's a worrisome leadership in North Korea," Mullen told ABC's "The View."

He called the authoritarian state's leader, Kim Jong-Il, "a very unpredictable guy, a very dangerous guy."

South Korean Prime Minister Kim Hwang-Sik also pointed to the succession process in North Korea, telling parliament Wednesday the shelling was designed to bolster the position of the leader's son and play up outside threats.

The North was trying "to brandish heir apparent Kim Jong-Un's military prowess, strengthen internal unity and vent internal discontent toward the outside", the premier told the National Assembly on Wednesday.

The United States has condemned the attack and vowed to stand behind its alliance with South Korea, but officials and military leaders have praised Seoul for showing "restraint" and made no threats of possible military action.

The United States and South Korea announced a joint naval show of force including an American aircraft carrier to deter the North, which killed a total of four people in its first shelling attack on civilians since the 1950-53 war.

But Pentagon spokesman Colonel Dave Lapan insisted the exercises in the Yellow Sea had been planned for some time and the date had been agreed before the artillery barrage this week.

"This had been planned and in the works," Lapan told reporters. "It was not a reaction to the North's unprovoked attack."

The US State Department said the artillery barrage of a South Korean island was "a clear premeditated action by North Korea specifically intended to inflame tensions in the region."

The United States acknowledged there was no guarantee that North Korea would back away from confrontation, and Washington was "prepared" for that scenario, State Department spokesman Philip Crowley told reporters.

"We believe we are taking the right approach and we would hope that over time North Korea would recognize and move in a different direction, but we understand that the track record suggests they won't, and we're prepared for that if needed," Crowley said.

He also said China had a pivotal role to play in defusing tensions and that Washington hoped Beijing would use its influence with North Korea.

"China does have influence with North Korea and we would hope and expect that China would use that influence first to reduce tensions that have arisen as a result of North Korean provocations, and then secondly continue to encourage North Korea to take affirmative steps to denuclearize," he said.