After conducting their fourth nuclear test and launching a long-range rocket last week, North Korea has expanded a uranium-enrichment plant and restarted a nuclear reactor that could produce plutonium in weeks. Pyongyang has expanded the Yongbyon uranium enrichment facility for the first time since shutting it down in 2007, according to the director of US National Intelligence, James Clapper.

The reactor was closed in an "aid-for-disarmament" agreement, but after its third nuclear test in 2013 the hermit nation began renovating it. Washington estimates that once in full flow, it could be capable of producing around six kilograms of plutonium in a year, which would be enough to build one nuclear bomb.

According to experts, the reactor could be ready to recover plutonium in a "matter of weeks or months". The findings could fuel further speculation that the isolated nation plans to develop long-range nuclear weaponry that could be target the US.

Recently, North Korea launched a rocket carrying an "Earth observation satellite" into space. There are fears that this technology could be used to help guide long-range ballistic missiles and it came just a few weeks after the Pyongyang claimed it successfully tested a "miniaturised" hydrogen bomb.

According to Reuters, Clapper told the Senate Armed Services Committee: "We assess that North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the plutonium production reactor.

"We further assess that North Korea has been operating the reactor long enough so that it could begin to recover plutonium from the reactor's spent fuel within a matter of weeks to months," he added.

It is not known if the DPRK has already developed nuclear weapons, but some experts believe that it could have up to 10 bombs.

US President Barack Obama has spoken with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-Hye, who all expressed a need for a "strong and united international response to North Korea's provocations, including through a robust UN Security Council Resolution," the White House said.