mainland and military bases after American stealth bombers flew training runs over South Korea.
The order came as US Defence Secretary Chuck Hagel, with tensions soaring on the Korean peninsula, said Washington would not be cowed by Pyongyang's bellicose threats and stood ready to respond to "any eventuality".
Kim directed his rocket units on standby at an overnight emergency meeting with top army commanders, hours after nuclear-capable US B-2 stealth bombers were deployed in ongoing US joint military drills with South Korea.
In the event of any "reckless" US provocation, North Korean forces should "mercilessly strike the US mainland... military bases in the Pacific, including Hawaii and Guam, and those in South Korea", he was quoted as saying by the official Korean Central News Agency.
While North Korea has no proven ability to conduct such strikes, Kim said: "The time has come to settle accounts with the US imperialists."
The youthful leader argued that the stealth bomber flights went beyond a simple demonstration of force and amounted to a US "ultimatum that they will ignite a nuclear war at any cost".
An unidentified South Korean military official quoted by Yonhap news agency said a "sharp increase" in personnel and vehicle movement had been detected at the North's mid- and long-range missile sites.
The US rarely acknowledges B-2 flights to the Korean peninsula, which remains technically at war. The aircraft, which dodge anti-aircraft defences, bombed targets in conflicts in Serbia, Afghanistan, Iraq and Libya.
The flights came as part of annual drills between the US and South Korea, which North Korea each year denounces as rehearsals for war.
Pyongyang has been particularly vocal this time, angered by UN sanctions imposed after its long-range rocket launch in December and the third nuclear test it carried out last month.
Kim's order formalised steps already taken by the Korean People's Army (KPA), which put its strategic rocket units at combat-ready status on Tuesday. The following day it cut the last remaining military hotline with South Korea.
The bulk of the threats emanating from Pyongyang have been dismissed as bluster, and North Korea has no confirmed missile capability to reach the US mainland - or indeed Guam or Hawaii in the Pacific.
But Washington has opted to match the threats with its own muscle-flexing.
"We will be prepared - we have to be prepared - to deal with any eventuality," Hagel told reporters at the Pentagon.
"We must make clear that these provocations by the North are taken by us very seriously and we'll respond to that," Hagel said, defending the B-2 deployment.
US military intelligence has noted that the North's warlike rhetoric has not, so far, been matched by any overtly provocative troop build-up.
Pyongyang has also been careful not to allow tensions to affect the Kaesong industrial complex, a joint South-North venture that provides the regime with crucial hard currency.
Present at the emergency meeting convened by Kim in Pyongyang were the KPA chief of general staff, director of operations and commander of strategic rocket operations.
KCNA provided an unusually precise timing for the meeting of 00:30 am (1530 GMT Thursday) in an apparent effort to underline the urgency and import of Kim's order.
But analysts warned against reading too much into what is the latest in a long series of incremental rhetorical upgrades.
"It shouldn't be taken to mean war is imminent," said Kim Yong-Hyun, a North Korea expert at Dongguk University.
"It's an inevitable and calibrated reaction to the B-2 deployment, and this who-blinks-first game with the United States will continue for a while yet," he said.
On the assumption that the North would never invite a full-scale conflict it would surely lose, experts believe it may opt for a limited provocation, similar to its 2010 shelling of a South Korean island that killed four people.
