The Institute for Science and International Security (ISIS) based in Washington published an update detailing North Korea's nuclear capabilities after Pyongyang announced on Tuesday that its main atomic weapons facility was fully operational.

ISIS said that the North could be developing "more sophisticated atomic weapons" at a new "hot cell" facility undergoing construction at the Nyongbyon site.

"The signatures visible through a historical analysis of satellite imagery are consistent with an isotope separation facility, including tritium separation," ISIS said in its report.

Key element

Tritium is a key element for developing advanced thermonuclear weapons, producing far greater yields than those using solely plutonium and uranium.

"Whether North Korea can make nuclear weapons using tritium is unknown although we believe that it remains a technical problem North Korea still needs to solve," the think tank said.

"Solving this problem would likely require more underground nuclear tests," ISIS added.

North Korea has carried out three nuclear tests between 2006 and 2013. The first two used plutonium devices, while the fissile material used in the third has yet to be confirmed, though it is widely believed that uranium was used.

'Quantity and quality'

Meanwhile, the director of Pyongyang's atomic agency announced on Tuesday that the North had made innovations to improve its nuclear arsenal "in quality and quantity as required by the prevailing situation," according to state-run KCNA news agency.

"If the US and other hostile forces persistently seek their reckless hostile policy towards the DPRK and behave mischievously, the DPRK is fully ready to cope with them with nuclear weapons at any time," KCNA quoted the director as saying, referring to the country by the acronym of its official title.

The US responded by urging Pyongyang to "focus instead on fulfilling its international obligations and commitments" to denuclearization, calling the director's statement "irresponsible provocation."

The rising tensions come ahead of the 70th anniversary of the ruling Workers' Party on October 10, when a massive military parade is expected to be held.

ls/jil (AFP, AP, Reuters)