The Democratic People's Republic of Korea (DPRK) on Monday accused South Korean President Lee Myung-bak of being engaged in a "plot" over his demand the DPRK apologise for the sinking of a warship.

Lee's speech was a plot to conceal his "inferior legerdemain (sleight of hand)" and the "conspiratorial farce" and "charade" showed Lee was a "crazy traitor" trying to block the unification of the nation, the official Korean Central News Agency (KCNA) quoted a National Defence Commission (NDC) spokesman as saying in a statement.

The spokesman said the speech was a violation of past agreements reached by the two countries, and that South Korea's refusal to receive NDC inspectors to verify the evidence it says it has of the North's involvement was to prevent its legerdemain from being exposed.

He said "the group of traitors" of South Korea could never dodge the responsibility for having staged the "farce" against the DPRK.

The Cheonan, a 1,200-ton South Korean warship with 104 crew members onboard, sank on March 26 near the maritime border with the DPRK after an explosion. Only 58 sailors were saved.

South Korea released the results of an investigation on May 20, accusing the DPRK of sinking the Cheonan in a torpedo attack.

The NDC issued a statement later on May 20, denying the accusation and offering to send inspectors to join the South Korean probe on the incident.

Lee delivered a speech on Monday, saying that his country would resort to measures of self-defense in case of further military provocation by the DPRK and demanding apology.