A video released by North Korea has depicted a nuclear attack on Washington, as an angry Pyongyang continued to hit out against joint military drills between South Korea and the United States.

The four-minute state media video - titled "Last Chance" - showed a digitally created scene of a missile being fired from a North Korean submarine and wiping out the US capital, the Associated Press news agency reported.

Pyongyang, which says the military drills are a dress rehearsal for an invasion, was angered when South Korean media reported that the exercises included training for a "decapitation strike" on the North's top leaders.

North Korean leader Kim Jong-un on Saturday warned that his military was prepared to attack Seoul's presidential palace unless South Korean President Park Geun-hye apologised for "treason" and executed officials Pyongyang accuses of planning to attack it.

The North is believed to have artillery capable of striking Seoul, though such a strike is said by analysts to be highly unlikely.

A video similar to "Last Chance" was aired in 2013 when North Korea also conducted a nuclear test and satellite launch.

Jong-un's government has been developing its nuclear weapons and missile capabilities, but is not believed to have perfected either enough to pose a credible threat to major US cities.