- Video report by ITV News Washington Correspondent Robert Moore
South Korea has simulated an attack on North Korea after Pyongyang tested a hydrogen bomb potentially more destructive than an atomic bomb.
South Korean media said Seoul's military believes the North is readying the launch of a ballistic missile in response to the live-fire attack.
The South Korea drill involved F-15 fighter jets and the firing of land-based "Hyunmoo" ballistic missiles, which officials said "accurately struck" a target in the sea off the country's eastern coast to "strongly warn" the North.
The United Nations Security Council will hold an emergency meeting later to discuss responses to the underground test Pyongyang hailed as a "perfect success" on Sunday.
The US has already warned any threats by North Korea will be met with a "massive military response" after the test on a weapon with an estimated strength of 50 kilotons.
- What action could the US take after the nuclear launch?
US President Donald Trump has been briefed on the "many military options" against North Korea, US Defence Secretary Jim Mattis confirmed.
Mr Mattis said the US is "not looking to the total annihilation" of North Korea, but added "we have many options to do so".
North Korea's nuclear test was its sixth since 2006 but the first since Mr Trump took office in January and defied his recent threats to leader Kim Jong-un.
Asked if he would launch an attack on Pyongyang, Mr Trump said only: "We'll see."
He tweeted that hindering the North through restrictions on its trading partners was among the options being considered.
- What action has Britain and other nations called for?
Foreign Secretary Boris Johnson said the acceleration of H-bomb testing "would unquestionably present a new order of threat".
He said "all options are on the table" but warned against Britain pursuing a military response.
On the test itself, Mr Johnson said: "There’s no question that this is another provocation, it’s reckless, what they’re doing."
Prime Minister Theresa May said the latest development poses an "unacceptable further threat to the international community".
A top South Korean official said president Moon Jae-in would discuss with the US ways to deploy the "strongest strategic assets" to isolate Pyongyang.
Donald Tusk, the president of the European Commission, called for the EU and UN to bring in yet more sanctions against the already highly isolated and financially crippled state.
"The stakes are getting too high," he added in a tweet.
- What has North Korea said about the bomb's power?
The blast came hours after Pyongyang issued images of Kim Jong-un inspecting the hydrogen bomb designed for ICBMs.
North Korean state TV later announced the "recent test" of a bomb with "unprecedentedly big power".
The device is estimated to be five times the strength of the Nagasaki bomb and nearly 10 times more powerful than its previous test.
"The perfect success in the test of the H-bomb for ICBM ... marked a very significant occasion in attaining the final goal of completing the state nuclear force," the announcer on state TV said.
Quake tremors from the blast were recorded at 12.29pm local time in the northern North Korean province of Hamgyong.
It was recorded as an artificial 5.6 magnitude quake by South Korea's weather agency and a 6.3 magnitude explosion by the US Geological Survey.
- How has North Korea ramped up testing and what has Trump threatened previously?
North Korea conducted two nuclear tests last year, the most recent in September, amid international alarm and condemnation led by the US.
The weapons tests include flight-testing developmental ICBMs and recently flying a ballistic missile over Japan as it bids to obtain a nuclear-armed missile.
Mr Trump warned North Korean leader Kim he faced "fire and fury" if he continued to pursue his nuclear ambitions.
In a written statement he said the "threatening and destabilising actions" only increased Pyongyang's isolation in the region and around the world.