North Korea may be close to completing an upgrade of its rocket launch site that would enable it to launch larger rockets.
The information was revealed by the US-Korea Institute at Johns Hopkins University in Washington referring to satellite images from recent months. The images, obtained by commercial operators including Airbus Defence and Space, show construction activity and new propellant bunkers at the launch pad and engine test stand at North Korea's Sohae Satellite Launch Station.
The institute said it could mean North Korea, known for its defiant stance against international rules, may be able to launch larger rockets as early as 2016.
North Korea states its rocket development is part of its legitimate efforts to develop the capability to launch satellites. However, analysts fear advancing its rocket technology would bring the communist country closer to another goal – developing an intercontinental ballistic missile capable of carrying an atomic warhead and reaching the USA.
Despite international sanctions, North Korea has tested three nuclear devices in the past. On Thursday, the communist country's dictator Kim Jong Un claimed North Korea has developed a hydrogen bomb, a step up from the less powerful atomic bomb. However, experts have questioned the veracity of his claims.
A hydrogen bomb, also known as a thermonuclear bomb, uses more advanced technology to produce a significantly more powerful blast than an atomic bomb. If the claims were right, it would mean North Korea is much closer to developing nuclear weapons.
However, the country has a reputation for making overblown claims about its capabilities. In 2010 North Korea claimed it had successfully developed fusion technology.
An official at South Korea's intelligence agency told Yonhap news agency there was no evidence that the North had hydrogen bomb capacity, and believed Kim was speaking rhetorically.
North Korea and South Korea are formally still at war as the 1950s Korean conflict only ended with a truce. Since then, North Korea has been repeatedly threatening to destroy the South as well as its major supporter the USA.
"I think it's unlikely that they have an H-bomb at the moment, but I don't expect them to keep testing basic devices indefinitely, either," said Jeffrey Lewis of the California-based Middlebury Institute of International Studies at Monterey.
It was possible the North was referring to the technology of boosting the yield of a nuclear device, possibly using fusion fuel, Lewis said.
Assessing progress of the North's nuclear programme is difficult because no one outside a close circle of leaders and experts knows what advances have been made.
North Korea also said in September that its main nuclear complex was operating and it was working to improve the "quality and quantity" of weapons which it could use against the United States at ‘any time’.
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