SEOUL (Reuters) - Mercurial North Korea's nuclear threat has reached an "alarming level" and it is now trying to miniaturise weapons to improve their mobility and impact, a South Korean government official said.
A U.S. think-tank has also said satellite images taken last week showed that construction or excavation activity was taking place at the North's main Yongbyon nuclear complex.
News that the North was pushing ahead with its nuclear plans in defiance of international pressure comes as Pyongyang, which has just recently set in motion a father-to-son power transition, has said it wants to return to stalled nuclear talks.
"We have judged that North Korea is currently operating all its nuclear programmes, including highly enriched uranium processing and the nuclear facility in Yongbyon," said Kim Tae-hyo, the president's secretary for national strategy, according to the JoongAng Daily on Wednesday.
Even though it has exploded nuclear devices, North Korea has not shown it has a working nuclear bomb. Experts say they do not believe it has the ability to miniaturise an atomic weapon to place on a missile.
Under an earlier aid-for-disarmament agreement, the reclusive North began to close down Yongbyon, which when fully operational can produce enough fissile material for one nuclear bomb a year.
Last year, Pyongyang announced that in the face of U.S. hostility it would restore parts of the plant, a year after blowing up the complex's cooling tower in what it said was a display of its commitment to nuclear disarmament.
"Their nuclear programme is evolving even now at a very fast pace," Kim told a forum on the future of Northeast Asia, adding the North's nuclear threat had reached an "alarming level."
North Korean Vice Foreign Minister Pak Kil-yon told the U.N. General Assembly last week that Pyongyang would bolster its "nuclear deterrent" in response to the threat posed by the United States, but promised never to use its atomic arsenal to attack or threaten any nation.
The Institute for Science and International Security in Washington said in a report that satellite images showed there was activity in the area surrounding the destroyed cooling tower.
"However, there is no indication in the imagery that North Korea is rebuilding its cooling tower," it said.
"In addition, the new excavation activity appears to be more extensive than would be expected for rebuilding the cooling tower. But the actual purpose of this excavation activity cannot be determined from the image and bears watching."
Until recently, there was no indication of new construction or excavation activity in the area of the destroyed cooling tower, the report said.
The Yongbyon complex consists of a five-megawatt reactor, whose construction began in 1980, a fuel fabrication facility and a plutonium reprocessing plant, where weapons-grade material is extracted from spent fuel rods.
The site, about 60 miles (100 km) north of Pyongyang, also houses a 50-megawatt reactor whose construction was suspended under a 1994 nuclear deal with the United States. That reactor is nowhere near completion.
U.S. officials said prior to the North's second nuclear test last year, it had produced about 110 lbs (l50 kg) of plutonium, which proliferation experts say would be enough for six to eight nuclear weapons.
"It is our belief that North Korea is constantly working on making their weapons smaller, as all nations with nuclear programs wish to do, in order to produce nuclear weapons with more firepower and less plutonium," Kim said.
"When the weapons are made mobile, they will be placed in the field, and when that time comes, they could wreak immense havoc on South Korean soil wherever they are aimed."
Editing by Miral Fahmy