North Korea vows to enhance nuclear deterrence in response to US subcritical nuclear test.

North Korea has vowed to take measures necessary for improving its nuclear deterrence posture after the US conducted a subcritical nuclear test, according to state KCNA news agency. The US Department of Energy's National Nuclear Security Administration conducted the experiment at its Nevada test site to collect data for nuclear warheads' reliability and effectiveness. North Korea's response indicates the test adds new tension to the international nuclear arms race.