North Korea said in January past year it had tested a miniaturised hydrogen bomb.
As President Donald Trump left a church service Sunday, he told a pool of reporters that "we'll see" if the USA attacks North Korea following its latest nuclear bomb test.
The White House will be under global pressure to deliver a coherent response to North Korea.
Mrs May added: "I discussed the serious and grave threat these unsafe and illegal actions present with President Abe in Japan this week and reiterate the call we jointly made for tougher action, including increasing the pace of implementation of existing sanctions and looking urgently in the UN Security Council at new measures". "I don't have any serious doubt in my mind that this is what the North Koreans say it is, and that's a thermonuclear weapon". "It will isolate them further".
The action suggested in Trump's trade tweet would be radical: The US imports about United States dollars 40 billion in goods a month from China, North Korea's main commercial partner.
Moscow said talks were the only way to resolve the crisis.
What does the test tell us?
"Many anchor women now are very young and handsome and are more suitable to appear before the viewers", she said.
This undated picture released by North Korea's official Korean Central News Agency (KCNA) on September 3, 2017 shows North Korean leader Kim Jong-Un (C) looking at a metal casing with two bulges at an undisclosed location. State media said the device could be loaded on to a ballistic missile.
Juche is North Korea's homegrown ideology of self-reliance that is a mix of Marxism and extreme nationalism preached by state founder Kim Il Sung, the current leader's grandfather.
TN Dalit student who moved SC against NEET ends life
The state government is yet to release the break-up of state and other board students who had got admissions based on NEET. According to reports, Anitha had secured 1176 marks out of 1200 in her Class XII State board examinations.
"But based on the seismic signature, the yield of this test definitely is an order of magnitude higher than the yields of the previous tests". Last week, after Trump said that "talking is not the answer", Defense Secretary James Mattis followed up by saying: "We're never out of diplomatic solutions".
The United States condemns this test and rejects the regime's claim that these tests-and these weapons-ensure North Korea's security.
The hydrogen bomb, also called a thermonuclear bomb, uses fusion - or atomic nuclei coming together - to produce explosive energy.
How can the world respond?
Worryingly, it also suggests that this is a programme that is progressing on all fronts at a faster rate than many had expected.
However many analysts believe that even a limited military strike on North Korea could be potentially catastrophic.
How did news of the test emerge?
According to Seoul, nuclear test on Sunday resulted in an artificial 5.7 magnitude quake in Kilju, which is in the northern Hamgyong province where North Korea conducts its nuclear tests, according to the AP.
The hydrogen bomb is the global standard for the five nations with the greatest nuclear capabilities: the US, Russia, France, the United Kingdom and China. His comments came just a few hours after he spoke with Japanese Prime Minister Shinzo Abe, a key ally in the region.
Just before Sunday's test, according to state media, Kim and the other senior leaders at the party presidium meeting discussed "detailed ways and measures for containing the USA and other hostile forces' vicious moves for sanctions".
Under third-generation leader Kim, North Korea has been pursuing a nuclear device small and light enough to fit on a long-range ballistic missile, without affecting its range and making it capable of surviving re-entry.
Beijing's response was predictable - condemnation, urging an end to provocation and dialogue. But it also spoke of urging North Korea to "face up to the firm will" of the worldwide community to see denuclearisation on the Korean peninsula.
There is no sign, though, that China is willing yet to see that "firm will" go beyond United Nations sanctions, which recently clamped down on seafood and iron ore exports, in addition to the coal and minerals that are already banned from crossing the border. Later on Sunday, Russian President Vladimir Putin is set to meet his Chinese counterpart Xi Jinping in China.