Donald Trump has condemned an apparent nuclear test by North Korea as "hostile and dangerous" after the secretive state said it had performed a "perfect" explosion of its most powerful weapon yet.
"Their words and actions continue to be very hostile and dangerous to the United States," Mr Trump said in a tweet hours after the blast caused condemnation around the world.
"North Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success," he added.
"South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!"
The US President's national security adviser Herbert Raymond McMaster spoke to South Korea about the crisis this morning.
World leaders have condemned the actions of Kim Jong-un's regime, with German Chancellor Angela Merkel and French President Emmanuel Macron describing the developments as a “new dimension of provocation”.
In a written statement, Mr Macron called for a "united and clear reaction" from the European Union and asked for "the members of the United Nations Security Council to quickly react to this new violation by North Korea of international law."
The French leader said the international community "must treat this new provocation with the utmost firmness" to bring North Korea back to the path of dialogue and give up its nuclear and missile programs.
Boris Johnson said "all options are on the table" after North Korea's latest nuclear test, but warned that "none of the miliary options are good".
The UK foreign secretary urged China to put tougher economic pressure on the Communist dictatorship to bring about a resolution to the escalating crisis on the Korean peninsula.
North Korea said it had tested a weapon which could be loaded on to a long-range missile, hours after seismologists detected an earth tremor.
Pyongyang claimed it had tested a hydrogen bomb, a device much more powerful than an atomic bomb.
In an announcement on state TV, North Korea said it had developed an advanced weapon of “great destructive power”.
A 6.3-magnitude earthquake was detected in the country shortly before 7am BST on Sunday, 75km (45 miles) north-northwest of Kimchaek, the location of previous tests.
Kim's state news agency released images apparently showing him laughing as he inspected a hydrogen bomb that will be loaded on a new intercontinental ballistic missile (ICBM).
The propaganda drive by North Korea's official KCNA news agency comes amid heightened regional tension following Pyongyang's test launch of two ICBM-class missiles in July that potentially had a range of about 10,000 km (6,200 miles) that could hit many parts of the mainland United States.
Under its current leader, North Korea has pursued work on building nuclear weapons and long-range ballistic missiles that can deliver them at an unprecedented pace, defying UN sanctions and international pressure.
Experts and officials have said North Korea could conduct its sixth nuclear test at any time, and that the reclusive country has maintained a readiness at its nuclear test site to conduct another detonation test at any time.
The hydrogen bomb's power is adjustable to hundreds of kilotons and can be detonated at high altitudes, with its home-produced components allowing the country to build as many nuclear weapons as it wants, KCNA news agency said.
Mr Kim visited the country's Nuclear Weapons Institute and "watched an H-bomb to be loaded into new ICBM", KCNA said.
"All components of the H-bomb were homemade and all the processes ... were put on the Juche basis, thus enabling the country to produce powerful nuclear weapons as many as it wants, he said."
Additional reporting by agencies.
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