North Korea has reached technological readiness to miniaturise nuclear warheads for ballistic missiles, the South Korean Ministry of National Defence (MND) has claimed.
In its 2014 edition of its biennial white paper seen by Yonhap News Agency, the MND states that Pyongyang appears to have achieved a 'significant level' of technology to miniaturise nuclear weapons but is yet to demonstrate such capability.
An unnamed South Korean MND official said: "We don't have any intelligence that North Korea completed the miniaturisation.
"In consideration of the fact that acquiring such technology takes around two to seven years in general and eight years have passed since the North conducted its first nuclear test, however, its capability for small nuclear warheads would have reached a significant level."
The paper also alleges that the country is believed to have '(missiles) capabilities that could threaten the US mainland, having fired off long-range missiles five times'.
However, the official added the missiles are yet to enter operational service in the North Korean military.
The white paper also stated: "North Korea is presumed to have secured some 40kg of weapons-grade plutonium by reprocessing spent nuclear fuel roads multiple times, and it is evaluated to have been working on the highly enriched uranium programme."
North Korea carried out its third and most recent nuclear test from Punggye-ri Nuclear Test Site, Kilju County, in February 2013.
Sharing details of Pyongyang's other military advancements, the MND said North Korea has established a new corps-scale military unit in its North Hamkyong Province to improve defences along the border and protect its military facilities.
The country is suspected of building new submarines types, including one capable of firing ballistic missiles, and continuing its military build-up through the procurement of additional armoured vehicles, rocket launchers and warships.
According to the paper, as of October 2014, North Korea had 1.2 million regular service personnel compared to South Korea's 630,000.