As South Korea commemorates the fifth anniversary of North Korea's deadly shelling of Yeonpyeong-do island, Pyongyang has issued a warning, saying it will retaliate "mercilessly" if the South fires at its waters in the West Sea.A spokesman for the regime's southwestern military unit said South Korea is scheming to attack North Korean waters again.In a separate report, the North's state-run Korean Central News Agency said if Seoul respects the deal signed in August to defuse military tensions, the South must behave accordingly.On November 23rd, 2010, North Korea fired scores of artillery rounds and rockets at the South Korean border island, killing two soldiers and two civilians.It was North Korea's first attack on South Korean territory since the Korean War.A memorial service commemorating the Yeonpyeong-do attack will take place later this Monday at the War Memorial in Seoul.