Pyongyang has slowly and steadily improved its missile capabilities in recent years and US officials say its missiles may be capable of hitting outlying US territories and states, including Guam, Alaska and Hawaii.
Some private experts say even this view is alarmist. There is no evidence, the officials say, that North Korea has tested the complex art of miniaturizing a nuclear weapon to be placed on a long-range missile, a capability the United States, Russia, China and others achieved decades ago.
In other words, North Korea might be able to hit some part of the United States, but not the mainland and not with a nuclear weapon.
The threats against the United States by North Korea's young leader Kim Jong-un are "probably all bluster," said Gary Samore, until recently the top nuclear proliferation expert on President Barack Obama's national security staff.
"It's extremely unlikely they have a nuclear missile which could reach the United States," said Samore.
The North Koreans "are not suicidal. They know that any kind of direct attack (on the United States) would be end of their country," said Samore, now at Harvard University's Kennedy School.
On Wednesday, North Korea's state-run KCNA news agency said its military had "ratified" an attack involving "cutting-edge smaller, lighter and diversified nuclear strike means," an apparent reference to miniaturized nuclear weapons.
It was the latest in a stream of invective from Pyongyang against what it apparently sees as hostile U.S.-South Korean military exercises, and U.N. sanctions imposed after its latest underground nuclear test.
Also on Wednesday, the Pentagon said it was moving a missile defense system known as the THAAD, or Terminal High Altitude Area Defense, to Guam, which Pyongyang has specifically threatened.
Bombs and missiles
The details of North Korea's weapons programs known to U.S. and other intelligence agencies remain classified. And there appear to be gaps in that knowledge, due to North Korea's highly secretive nature.
Some US official and private weapons experts say North Korea may have succeeded in designing, and possibly building, a miniaturized nuclear device that could be fit aboard medium-range missiles known as the Nodong.
This is in dispute, however. And even if Pyongyang has developed such a warhead, there are serious doubts about whether North Korea would be able to test it enough to ensure it actually worked.
Medium-range missiles such as the Nodong might be able to reach U.S. allies South Korea and Japan, as well as Okinawa, where there is still a large U.S. military presence. But they do not have the range to hit even remote U.S. Pacific territories.
Another missile that U.S. intelligence agencies are watching closely is the KN-08, which has a longer range than the Nodong, and was first shown off in a North Korean military parade a year ago.
Last month, Admiral James Winnefeld, vice chairman of the Joint Chiefs of Staff, told journalists that "we believe the KN-08 probably does have the range to reach the United States."
On Thursday, a U.S. official, speaking on condition of anonymity, said the United States believes the KN-08 could hit U.S. Pacific territory Guam, Hawaii and Alaska, but not the U.S. mainland.
Another official acknowledged that the U.S. estimate of the missile's range is based on limited intelligence.
Greg Thielmann, a former State Department intelligence official now with the Arms Control Association, was more skeptical of the KN-08 missile.
When some experts examined close-up pictures of a KN-08 missile on display in Pyongyang, they concluded it was a fake or mock-up, Thielmann said.
"This was not ready for prime time," he said.
On Thursday, Western officials confirmed reports that North Korea had moved yet another weapon, apparently a medium-range missile known as a Musudan or Nodong B, to its east coast. Experts said it was unclear if the missile was moved as a menacing gesture or in preparation for a test firing.
The Musudan is believed to have a range of 3,000 kilometers (1,875 miles), more than the Nodong, which would put all of South Korea, Japan and possibly Guam in its range.
Thielmann said the Musudan had not flown either. "A missile that has never even had a flight test is not an operational system and is not a credible threat," he said.
U.S. officials say the latest threats about a nuclear strike go beyond previous rhetoric voiced not only by Kim Jong-un, but also his father Kim Jong-il and grandfather, North Korea's founding dynast Kim Il-sung.
One U.S. official said that until now, read closely, the threats from Pyongyang were conditional, suggesting an attack on the United States would only take place if the U.S. first acted against North Korea.
However, that was not the case with Wednesday's threat of a nuclear strike, the official said.
Thielmann said North Korea has several hundred missiles, most of which are variants of the SCUD-B and SCUD-C missiles with a range of 300 km and 500 km (187 to 312 miles). They have some dozens of Nodong missiles with an estimated 1300 km (862 miles) range.
"Neither of these (medium range) systems can reach Guam. Neither of them can reach Hawaii or (the) Aleutian Islands," Theilmann said. There are "missiles that North Korea pretends that it has and ... various people in the United States seem to want to lend credence to these fantastic North Korean claims."