South Korea launched a ballistic missile exercise late on Sunday in response to Pyongyang's provocative detonation, state news agency Yonhap reported. "S. Korea's military stages ballistic missile exercise in response to N. Korea's nuke test," the agency said.
The nuclear weapon test registered with international seismic monitoring agencies as a man-made earthquake of magnitude 6.3, making it 10 times more powerful that North Korea’s last nuclear test a year ago.
Hours before the explosion, the North Korean state news agency had released pictures of Kim inspecting a silver-coloured, hourglass-shaped warhead, which was markedly different from a ball-shaped device detonated last year.
Experts said it indicated a two-stage thermonuclear device, suggesting Pyongyang has made significant progress towards its goal of a long-range nuclear missile.
The UN Security Council will hold an emergency meeting on Monday to discuss the latest development and possible responses.
Mr Johnson said: “It’s certainly our view that none of the military options are good...the distance between North Korea and Seoul is very, very small and they could basically vaporise large parts of the South Korean population even with conventional weapons, so that’s not really very easy to threaten or to deliver.”
Calling on China to take stronger action against its client state, he went on: “We have to consider how to respond and it’s our view in the UK, overwhelmingly that peaceful, diplomatic means are the best and we think the sanctions route still holds potential.
“China is responsible for 90 per cent of North Korea’s trade and North Korea only has six months of oil supplies left. There is scope to continue to put pressure on the regime."
Theresa May, meanwhile, called on the United Nations “urgently” to look at fresh sanctions.
The hydrogen bomb test, the sixth to be carried out by Kim and the first since Mr Trump became president, follows a series of long-range missile tests that pose an increasing threat to the US, and comes just days after a missile was launched that flew over Japan.
Mr Johnson said: “They seem to be moving closer towards a hydrogen bomb which, if fitted to a successful missile, would unquestionably present a new order of threat.”
President Vladimir Putin of Russia and President Xi Jinping of China met at a pre-planned summit in Xian, where they agreed to adhere to "the goal of denuclearisation on the Korean Peninsula”.