Moon Jae-in, South Korea's President, called for the "strongest possible" response to the nuclear test, including new UN Security Council sanctions to "completely isolate" the North.

Seoul and Washington also discussed deploying US strategic military assets to the Korean peninsula, South Korea's national security adviser Chung Eui-yong said in a news briefing.

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

A spokesman said US National Security Adviser HR McMaster spoke with Mr Chung, his South Korean counterpart, for 20 minutes in an emergency phone call about an hour after the detonation.

China's Foreign Ministry urged North Korea to stop its "wrong" actions.

The ministry said in a statement on its website that China resolutely opposed and strongly condemned North Korea's actions, and urged the country to respect UN Security Council resolutions.

Japanese and South Korean officials said an earthquake detected near the North's test site was around 10 times more powerful than previous detonations, and concluded the North had conducted its sixth nuclear test.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

It was the North's first nuclear test since US President Donald Trump took office, and marked a direct challenge to Mr Trump, who hours earlier had talked by phone with Japanese Prime Minister Shinzo Abe about the "escalating" nuclear crisis in the region.

North Korea said in an announcement on state television that a hydrogen bomb test ordered by leader Kim Jong-un was a "perfect success" and a "meaningful" step in completing the country's nuclear weapons programmes.

The bomb was designed to be mounted on its newly developed intercontinental ballistic missile (ICBM) the North said in the announcement, which came hours after the US Geological Survey (USGS) reported a 6.3 magnitude quake.