North Korea threatens action after US conducts subcritical nuclear test

North Korea said a recent subcritical nuclear test by the United States adds new tension to the international nuclear arms race and vowed to take measures necessary for improving its nuclear deterrence posture, the state-controlled KCNA news agency has said.

The US Department of Energy has said its National Nuclear Security Administration conducted a subcritical experiment at its Nevada test site to collect data to support the reliability and effectiveness of nuclear warheads.

"To cope with the strategic instability in the region and the rest of the world caused by the U.S. unilateral action, (we) cannot but reconsider the measures necessary for the improvement of the overall nuclear deterrence posture within the range of its vested sovereign right and possible options," KCNA said on Monday.

It did not elaborate on what measures it would consider.

Half-dozen nuclear tests

Subcritical nuclear tests do not involve a nuclear explosion and do not trigger a fissile chain reaction.

The United States is a signatory to the global nuclear test ban treaty and ended nuclear explosion tests in 1992.

North Korea has conducted six underground nuclear tests and is believed to be ready for a seventh, although South Korean officials have said there are no indications of an imminent test.

North Korea is under UN Security Council sanctions for its nuclear tests and for ballistic missile development.