Heads have started rolling in the aftermath of the violent eruption of hostilities on the Korean Peninsula two days ago.

South Korea's defense minister Kim Tae-young resigned late on Thursday as criticism mounted over the government's response to the North Korean shelling on a dispute island which killed four people.

Parliamentarians, military leaders and the opposition party had clamored for his head following the first attack on the South Korea-controlled civilian land. The North's artillery fire on the Yeonpyaong Island in the Yellow Sea on Tuesday had triggered fears of a full-blown war braking out on the Peninsula which has witnessed uneasy peace for more than five decades.

The presidential chief of staff Yim Tae-hee also said on Thursday the president's aide in charge of defense affairs has also been removed from his position.

North Korea denied on Tuesday it attacked first. The South Korean enemy, despite our repeated warnings, committed reckless military provocations of firing artillery shells into our maritime territory near Yeonpyeong island beginning 1pm (1500 AEDT), the AFP quoted a statement from the North's military supreme command.

The two Koreas are still technically at war as the Korean War had ended with an armistice, not a full peace treaty. Tensions between the nuclear-armed North and its neighbor who is economically far superior, have been intermittent. Earlier this year, the two countries were involved in a nasty bust-up as South Korea accused the North of sinking its warship, killing 46 sailors. The angry North responded with more aggression, carrying out more nuclear tests and building a new uranium enrichment plant.

The US, which has stationed tens of thousands of troops in South Korea, asked North's mentor and ally Beijing to reign in its belligerent ward. China does have influence with North Korea and we would hope and expect that China will use that influence, a State Department spokesman said.