South Korean media says Seoul's military believes North Korea is readying the launch of a ballistic missile, possibly an intercontinental version (ICBM).
CNN's Will Ripley later reported that North Korea had threatened to reduce its southern neighbour "to ashes".
Yonhap news agency reports that Seoul's defence ministry also measured North Korea's nuclear test on Sunday at 50 kilotons.
The detonation was the strongest ever from the North, which claimed the test was of a hydrogen bomb.
Events surrounding the ongoing nuclear crisis changed rapidly over the weekend - here's where the main actors currently stand.
North Korea claimed "perfect success" in an underground test of what it called a hydrogen bomb - potentially vastly more destructive than an atomic bomb.
It was the North's sixth nuclear test since 2006.
It was almost certainly intended to get under the skin of one man in particular - President Donald Trump.
North Korea has made a stunning jump in progress for its nuclear and missile program since Kim rose to power following his father's death in late 2011. The North followed its two tests of Hwasong-14 ICBMs, which, when perfected, could target large parts of the United States, by threatening to launch a salvo of its Hwasong-12 intermediate range missiles toward the US Pacific island territory of Guam in August.
It flew a Hwasong-12 over northern Japan last week, the first such overflight by a missile capable of carrying nukes, in a launch Kim described as a "meaningful prelude" to containing Guam, the home of major US military facilities, and vowed to launch more ballistic missile tests targeting the Pacific.
South Korea responded to the nuclear test with live-fire drills off its eastern coast Monday that were meant to simulate an attack on the North's main nuclear test site, reports the Associated Press.
The country's military said its live-fire exercise was meant to "strongly warn" Pyongyang. The drill involved F-15 fighter jets and the country's land-based "Hyunmoo" ballistic missiles firing into the Sea of Japan.
The target was set considering the distance to the North's test site and the exercise was aimed at practicing precision strikes and cutting off reinforcements, Seoul's Joint Chiefs of Staff said.
Each new North Korean missile and nuclear test gives Pyongyang's scientists invaluable information that allows big jumps in capability. North Korea is thought to have a growing arsenal of nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs.
THE UNITED STATES
US Defence Secretary, Jim Mattis, responded to North Korea's latest nuclear test by saying threats to the United States and its allies "will be met with a massive military response".
Mattis spoke at the White House on Sunday following a meeting with President Donald Trump and national security advisers. He says any response will be "both effective and overwhelming."
Mattis says the United States is "not looking to the total annihilation" of North Korea, but added "we have many options to do so."
Trump sent a string of tweets on Sunday both condemning the move and criticising the responses of China and North Korea.
Later, when asked by a reporter if the US would attack North Korea, Trump simply said: "We'll see.
THE UNITED NATIONS
The UN Security Council will hold an emergency meeting later today.
The US, Japan, France, Britain and South Korea requested the meeting after North Korea detonated what it called a hydrogen bomb.
It will be the Security Council's second urgent session in under a week on the North's weapons tests, which have continued in the face of a series of sanctions.
After North Korea launched a ballistic missile over Japan, the council Tuesday strongly condemned the test and reiterated demands that Pyongyang halt its ballistic missile and nuclear weapons programs.
Monday could bring additional condemnation and discussion of other potential steps.
Meanwhile, UN Secretary-General Antonio Guterres condemned Sunday's nuclear test. His spokesman calls it "profoundly destabilising for regional security."
A publication of the ruling Communist Party has urged China to avoid imposing a full embargo on North Korea.
The Global Times newspaper said in an editorial Monday that such a response would trigger war.
The paper said the nuclear test conducted Sunday was "another wrong choice that Pyongyang has made" in violation of U.N. Security Council resolutions.
On Sunday, Xi met on the sidelines of the BRICS Summit with Russian President Vladimir Putin and discussed North Korea's latest nuclear test, which has cast a shadow over an international summit hosted by Pyongyang's only major ally, China.
The official Xinhua News Agency said they agreed to "appropriately deal with" it, without elaborating.
The leaders of South Korea and Japan have agreed to work together to build support for further sanctions against North Korea following its latest nuclear test.
Japanese broadcaster NHK says Japanese Prime Minister Shinzo Abe and South Korean President Moon Jae-in discussed the crisis by telephone Monday, ahead of an emergency UN Security Council meeting.
Abe also spoke with President Donald Trump and Russian leader Vladimir Putin late Sunday night.
Japan's Foreign Ministry said that Abe strongly encouraged Russia to respond constructively as a permanent member of the Security Council. He and Putin agreed to continue talks later this week in Vladivostok, Russia.
Abe told Trump that North Korea's nuclear test is a serious threat to Japan's security that poses a "head-on challenge" to the international community.
Russian President Vladimir Putin has spoken by telephone with Japanese leader Shinzo Abe and urged restraint in responding to North Korea's claim to have set off a hydrogen bomb test.
Putin, in China for a meeting of leaders of the BRICS economic bloc, called Abe on Sunday.
Kremlin spokesman Dmitry Peskov told journalists that Putin "said the international community could not give in to emotions, should act calmly and deliberately, and stressed that the complex settlement of the nuclear and other problems of the Korean Peninsula can be achieved exclusively through political and diplomatic means".
Foreign Secretary, Boris Johnson, has warned Trump that military action against North Korea would lead to Kim Jong-un "vaporising" the people of South Korea.
While he condemned the "reckless" act he added that "all options are on the table, but we really don't see any easy military solution".