North Korea's military put its "strategic" rocket units on a war footing today, with a fresh threat to strike targets on the US mainland, Hawaii and Guam, as well as South Korea.

The move came as South Korea marked the third anniversary of the sinking of its naval vessel "Cheonan" by what Seoul insists was a North Korean submarine.

"All artillery troops including strategic rocket units and long-range artillery units are to be placed under class-A combat readiness," the Korean People's Army (KPA) supreme command said in a statement.

The units should be prepared to attack "all US military bases in the Asia-Pacific region, including the US mainland, Hawaii and Guam" and South Korea, said the statement carried by the Korean Central News Agency.

Despite a successful long-range rocket launch in December, most experts believe North Korea is years from developing a genuine inter-continental ballistic missile that could strike the mainland United States.

Hawaii and Guam would also be outside the range of its medium-range missiles, which would be capable, however, of striking US bases in South Korea and Japan.

North Korean leader Kim Jong-Un has spent the past few weeks touring frontline military units, monitoring live fire artillery drills and making inflammatory speeches about wiping out the enemy.

Sabre-rattling and displays of brinkmanship are nothing new in the region, but there are concerns that the current situation is so volatile that one accidental step could escalate into serious conflict.

"We are closely monitoring the situation. So far there has been no particular North Korean troop movement," a South Korean defence ministry spokesman said.

Addressing a ceremony for the 46 sailors who died in the 2010 "Cheonan" incident, South Korean President Park Geun-Hye warned Pyongyang that its only "path to survival" lay in abandoning its nuclear and missile programs.