North Korea said on Tuesday it was South Korea which fired first and that they had to fire in retaliation.  

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.

It added that the North's military will continue to make merciless military attacks with no hesitation if the South Korean enemy dares to invade our sea territory by 0.001 mm.

The authorities in the North said in the West Sea (Yellow Sea), there will only exist the sea boundary set by us, according to the report.

At least two South Korean soldiers have been killed and 19 others are injured as North Korea fired artillery shells at a South Korean Yeonpyaong Island in the Yellow Sea on Tuesday.

Meanwhile, Washington strongly condemned North Korea's attack on a South Korean island.

The United States strongly condemns this attack and calls on North Korea to halt its belligerent action and to fully abide by the terms of the Armistice Agreement, White House spokesperson Robert Gibbs said in a statement.