North Korean leader Kim Jong-un has ordered the country's rockets to be on stand-by targeting the US and South Korea following the flight of two American stealth bombers over the Korean Peninsula.
The state-run Korean Central News Agency (KCNA) said: "[Kim] convened an urgent operation meeting on the Korean People's Army's Strategic Rocket Force's performance of duty for firepower strike at the Supreme Command at 00:30 Friday."
The North Korean leader said the time has come to "settle accounts" with the US. North Korea views that the recent flight of nuclear-capable B-2s, as part of a military drill, has gone "beyond the phase of threat and blackmail".
Once again stepping up the threats against the US and South Korea, Kim said: "If they make a reckless provocation with huge strategic forces, the Korean People's Army should mercilessly strike the US mainland, their stronghold, their military bases in the operational theatres in the Pacific, including Hawaii and Guam, and those in South Korea."
North Korea possesses Soviet-era scud missiles, capable of striking Seoul and US bases, but its long-range missiles are yet to be tested. Although North Korea claims the long-range missiles can strike the US mainland, many experts believe the defiant nation does not have enough technical capability to carry out a strike.
The KCNA said: "Kim declared the revolutionary armed forces of the DPRK would react to the US nuclear blackmail with merciless nuclear attack, and war of aggression with an all-out war of justice."
Reports from Seoul also suggest that troop and vehicular movements have been detected at North Korea's missile launching sites.
"Sharply increased movements of vehicles and soldiers have been detected recently at North Korea's mid and long-range missile sites," a South Korean military official told the Yonhap news agency.
South Korea has kept a close watch on all short-range, mid-range and long-range missile launch sites in North Korea.
US Secretary of Defence Chuck Hagel said Washington is prepared for "any eventuality".
"I think their very provocative actions and belligerent tone have ratcheted up the danger. And we have to understand that reality," said Hagel during a press conference.
He also conceded that "there are lot of unknowns" pertaining to the secretive nation.