South Korea has said it will consider deploying the most powerful US tactical weapons after North Korea claimed to have detonated a hydrogen bomb in its sixth nuclear test.
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.
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.
Additional reporting by agencies
