North Korea is in the process of developing a new missile system for ships or submarines, according to a report in Jane's Defence Weekly.
N Korea has already proved it has missiles, by publicly testing them
Such a system could "fundamentally alter the missile threat" posed by Pyongyang, as it would then be able to target the entire US, the report says.
A companion land-based missile is thought to have been developed already.
The systems are based on the now decommissioned Soviet R-27 submarine-launched ballistic missile.
The report, published in the authoritative Jane's Defence Weekly, says the land-based system has an estimated range of 2,500km to 4,000km (1,500 miles to 2,500 miles) while the sea-based system is thought to be capable of hitting a target more than 2,500km away.
Map shows range of Taepodong 1 missile, flown over Japan in 1998. Range 1,500-2,000 km, payload: 1,000 kg
Evidence that North Korea preparing flight test of Taepodong 2. Range up to 8,000 km (could reach western US)
Evidence from Jane's Defence of a pair of new ballistic missiles - one sea-based
Other missiles: Scud-B: Range 300 km, payload 1,000 kg
Scud-C: Range 500 km, payload 7600-800 kg
Scud-D (Nodong): Range 1,000-1,300 km, payload: 700-1,000 kg
"These new land and sea-based systems appreciably expand the ballistic missile threat presented by the DPRK [North Korea]," the report said.
But the most significant part of the new developments appears to be the sea-based missile, as it could be transported almost anywhere in the world by submarine or ship.
Such a system "could finally provide [the North Korean] leadership with something that it has long sought to obtain - the ability to directly threaten the continental US," the report warns.
Ian Kemp, the news editor of Jane's Defence Weekly, said only five other countries have this capability: the declared nuclear powers of the US, the UK, France, China and Russia.
He told BBC News Online that North Korea was "almost certainly" developing the missile with the intention of adding nuclear warheads.
Pyongyang could also be intending to sell its new missiles to another country, although according to Jane's, there is no evidence that any attempt has been made so far.
Iran would appear the ideal customer, the Jane's report says, "given its requirement for a system capable of striking Israel from the security of its own territory".
A spokeswoman for the US state department declined to comment specifically on the apparent new threat.
Darla Jordan told BBC News Online simply: "The US will continue to work closely with other like-minded countries to address North Korea's nuclear efforts."
Information about North Korea's military capabilities is notoriously sketchy, given the ultra-secretive nature of its communist regime.
The US and North Korea remain deadlocked over Pyongyang's controversial nuclear programme, and the isolated Stalinist nation has long been seen as a threat to regional security.
North Korea has already proved it owns short-range missiles such as the Taepodong 1, which is capable of reaching a target up to 2,000km away.
A Taepodong 1 was fired over Japanese territory in 1998, landing in the Pacific Ocean and causing much alarm in Tokyo.
Correspondents say there is mounting evidence that Pyongyang has also been working on a missile with a much longer range, the Taepodong 2, which is thought to be capable of reaching targets 8,000km away, such as Hawaii or Alaska.
According to Mr Kemp, there is no evidence that the advent of the new missiles will affect the completion of the Taepodong 2.
The two systems could be complementary, he said.