North Korea is making progress on an intercontinental ballistic missile capable of delivering a first-generation nuclear warhead to the continental United States, a leading US think-tank said Tuesday.

The closely followed 38 North website of the Johns Hopkins University's US-Korea Institute argued that ICBM mock-ups seen at recent military parades in Pyongyang were "less fake" than originally believed.

Numerous experts had widely ridiculed the models of the North's road-mobile KN-08 ICBM seen in 2012 and July this year, with at least one respected aerospace engineer labelling them technically preposterous and a "big hoax".

An analysis posted by 38 North disagreed, saying they were consistent with the ongoing development of a missile with a limited intercontinental ability using only existing North Korean technology.

"Elegant or not," the mockups suggest an ability to assemble components and technologies to produce missiles with theoretical ranges of 5,500 kilometres (3,400 miles) to more than 11,000 km.

"Almost all of the configurations examined would be able to deliver a light, first-generation nuclear warhead at least as far as Seattle," it said.

The analysis was co-written by non-proliferation expert Jeffrey Lewis and aerospace engineer John Schilling.

The authors noted that glaring discrepancies in KN-08 mock-ups displayed in 2012 had largely disappeared by the time of the July parade.

And the new arrangement of welds and rivets was similar to that seen on recovered debris from the North's Unha-3 carrier, which successfully placed a satellite in orbit in December last year.

In a separate, technical paper published in Science and Global Security, Schilling stressed that the KN-08 was still very much a missile in development.

"The lack of flight testing strongly suggests that operational deployment is still months or years in the future," the engineer said.

And even with a successful test programme, it would likely be unreliable, limited in mobility and performance, and available only in small numbers, he added.

Lewis and Schilling's paper referenced recent analysis of satellite imagery indicating that North Korea was upgrading its main missile launch site, possibly to cater to larger, mobile weapons.

South Korea's Defence Intelligence Agency told parliament on Tuesday that North Korea had conducted five tests of long-range rocket engines this year.

An initial test of the KN-08 could come "at any time", Schilling said.

Missile delivery has often been cited as the main weakness of the North's nuclear weapons programme which, after three tests, is believed to be close to mastering the key technology of warhead miniaturisation.

December's satellite launch caused serious concern, but experts stressed that it lacked the re-entry technology needed to bring an ICBM down onto a target.

Nevertheless, Lewis and Schiller said dismissing the mock-ups paraded in Pyongyang would be dangerous.

"The simplest explanation here is that the (KN-08) is exactly what it appears to be: A developmental road-mobile ICBM of limited capability but still able to threaten the continental United States," they said.