South Korea has admitted it fired artillery shells that triggered an early morning clash with North Korea. However it says it was part of a military drill and denied it was directed at the North. Earlier Seoul blamed the North for what it called an unprovoked shelling of its island. Pyongyang claimed the South had violated its maritime border during the military drills. Seoul has since threatened its neighbour with what it called 'enormous retaliation'. Russia's foreign ministy said the clashes were unacceptable and called on both sides to show restraint.