North Korea has confirmed it has restarted plutonium production at its Yongbyon nuclear reactor facility after reports of increased activity.
The agency running Yongbyon was quoted by Japan's Kyodo News Agency as saying nuclear tests would continue as long as the US posed a threat.
The North restarted Yongbyon in June, UN inspectors confirmed.
The country has conducted four nuclear weapons tests and several missile tests breaching international sanctions.
The Atomic Energy Institute, which controls Yongbyon, said in a written interview with Kyodo: "We have reprocessed spent nuclear fuel rods removed from a graphite-moderated reactor".
The institute also told Kyodo that North Korea was producing highly enriched uranium, an essential component in nuclear power and weapons. The amount of plutonium and enriched uranium in North Korea is unknown.
"Under conditions that the United States constantly threatens us with nuclear weapons, we will not discontinue nuclear tests," the institute said.
International experts noticed activity at the site in recent months from satellite imagery.
The nuclear reactor at Yongbyon had been shut down in 2007.
UN monitors from the International Atomic Energy Agency have had no access to North Korea since being thrown out in 2009, so they rely largely on satellite data.
At North Korea's most recent ruling party congress, leader Kim Jong-un reinforced his policy of economic development coupled with a strong nuclear programme.
The US recently agreed to install an anti-missile system in South Korea that will be deployed solely to counter the threat from Pyongyang.
Yongbyon nuclear complex
- Reactor was shut down in July 2007 as part of a disarmament-for-aid deal
- International inspectors banned in April 2009 when North Korea pulled out of disarmament talks
- A uranium enrichment facility was revealed in 2010. An American nuclear scientist said centrifuges appeared to be primarily for civilian nuclear power, but could be converted to produce highly enriched uranium bomb fuel
- In 2013, North Korea said it would restart the nuclear reactor, the same year it conducted a nuclear test. It is believed to have shut it down for a period in 2014.
- Experts believe that reactor could make one bomb's worth of plutonium per year