North Korea's deadly strikes on a South Korean island were likely meant to bolster the military credentials of its little known leader-in-waiting, South Korea said Wednesday.

The artillery attack came two months after Kim Jong-Un, the youngest son of the current leader, consolidated his role by becoming a four-star general and vice chairman of the ruling party's Central Military Commission.

South Korean Prime Minister Kim Hwang-Sik told parliament the shelling of a Yellow Sea island, which claimed four lives and wounded 18 people, was a "premeditated, meticulously planned provocation".

The North was aiming "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.

Defence Minister Kim Tae-Young, after talks with US counterpart Robert Gates, also said "our judgment is that North Korea carried out the attack to consolidate the succession process by showing off Kim Jong-Un's leadership".

Kim and Gates agreed in a phone conversation that the attack was a "meticulously pre-planned provocation," the defence ministry said.

They also agreed that the two allies should step up their joint defence posture, including with US surveillance, in order to prevent further provocative acts by the North and an escalation.

The attacks came after the North sparked fresh fears this month over its nuclear programme by showing to an American scientist a modern uranium enrichment plant in Yongbyon outside the capital Pyongyang.

"We judged that after revealing the new uranium enrichment facility on November 12, North Korea made the artillery attack to give Kim Jong-Un the status of a strong leader," Yonhap news agency quoted the minister as saying.

The minister also said his military was determined to "respond resolutely" if the North makes any further provocations, and he vowed to strengthen defences on the frontline Yeonpyeong island that was targeted.

The South will deploy more K-9 self-propelled guns on top of the six already based there, replace 105-mm howitzers with self-propelled guns with longer ranges and bolster a tank contingent there, he said.

Yonhap news agency, quoting defence ministry sources, meanwhile said the artillery on the North's southwestern coast and long-range guns were maintaining a ready-to-fire posture on Wednesday.

It also gave more details of North Korean military activity Tuesday.

North Korean Mig-23 jet fighters took off from Bukchang base for reconnaissance near the border before the shelling began, and they were later redeployed further south to Hwangju airbase, the source said.

The North had also deployed ground-to-ship missiles and put their warships in battle position, according to the report.

During the South Korean parliament session, some lawmakers criticised the military for showing what they labelled hesitation when they returned fire, but Kim defended the military's response.

Lawmaker Kim Hak-Song of the conservative Grand National Party said the South should have used jet fighters to bomb the positions from which the shells were launched.

The defence minister said: "We also thought about that, but we chose not to use the air force for the sake of preventing escalation."