South Korea has been put on high alert after North Korea declared that its military units are in "No 1 combat readiness posture" targeting the US mainland, Hawaii and other American bases in the region as well as South Korea.

This is the first time North Korea has used the "combat readiness" term heightening the existing tensions in Korean Peninsula to a level closer to a war-like situation.

Ministries in South Korea have been placed on full preparedness as the latest threats coincide with the third anniversary of a naval attack on South Korea's Cheonan warship which killed several sailors.

"From this moment, the Supreme Command puts all of its field artillery including strategic rocket units and long-range artillery units into the No 1 combat ready posture," said the Korean People Army's Supreme Command, according to the state-run Korean Central News Agency (KCNA).

Statements from South Korea's supreme command of the army are extremely rare although North Korea resorts to frequent threats.

According to reports, South Korea considers the latest threat to be serious although officials have so far not detected any troop movement inside North Korea.

Referring to the latest drill by a B-52 bomber, North Korea said: "The US nuclear war racket has gone beyond the danger line and entered the phase of an actual war, defying the repeated warnings from the army and people of the Democratic People's Republic of Korea (DPRK)."

The statement added: "The present South Korean puppet authorities tried to link the Cheonan sinking case with the DPRK and shift the blame for the Yeonpyong Island shelling on to the DPRK. They should be mindful that everything will be reduced to ashes and flames the moment the first attack is unleashed."

China, North Korea's key ally, has once again appealed to all parties to show restraint in the "complex and sensitive situation."

The latest threat from North Korea comes hours after South Korean President Park Geun-hye urged North Korea to give up its nuclear weapons.

"North Korea must immediately abandon its thoughts that nuclear weapons will protect its regime. The only way North Korea will survive is if it voluntarily lays down its nuclear weapons, missiles, provocations and threats, and transforms into a responsible member of the international community," said Park, who recently took over as the first woman president of South Korea.