Image copyright AP Image caption N Korea's leader has vowed to develop nuclear weapons

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.

Monitors barred

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.

Image copyright SPL Image caption IAEA relies on satellite images of the site, like this one from 2004

Image copyright Reuters Image caption This image from 2008 shows Yongbyon before the main cooling tower was demolished when the reactor was taken offline

Yongbyon nuclear complex