US, South Korea Speak After NKorea's Nuclear Test

South Korea's presidential office says the security chiefs for Seoul and Washington have spoken following North Korea's sixth nuclear test.

The office says U.S. National Security Adviser H.R. McMaster spoke with his South Korean counterpart, Chung Eui-yong, for 20 minutes in an emergency phone call about an hour after the detonation.

North Korea's nuclear test Sunday was apparently its most powerful yet. State-controlled media say it was a hydrogen bomb. South Korea's weather agency says the apparent detonation set off a magnitude 5.7 earthquake, making the blast five to six times stronger than the North's fifth test in September 2016.