North Korea is preparing to launch a ballistic missile, possibly an ICBM, South Korean military sources said.
Seoul‘s defence ministry also measured North Korea’s nuclear test at 50 kilotons, Yonhap news agency reported.
The detonation on Sunday was the strongest ever from the North, which claimed the test was of a hydrogen bomb.
South Korea responded to the nuclear test with live-fire drills off its eastern coast on Monday that were meant to simulate an attack on the North’s main nuclear test site.
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 said Japanese prime minister Shinzo Abe and South Korean president Moon Jae-in discussed the crisis by telephone on Monday, ahead of an emergency UN Security Council meeting.
Mr Abe also spoke with President Donald Trump and Russian leader Vladimir Putin late on Sunday night.
The live-fire exercise was carried out to “strongly warn” Pyongyang over its claimed test of a hydrogen bomb, Seoul’s joint chiefs of staff (JCS) said.
The drill involved F-15 fighter jets and South Korea’s land-based “Hyunmoo” ballistic missiles, and the released live weapons “accurately struck” a target in the sea off the country’s eastern coast, the JCS said.
It came after Kim Jong Un’s regime on Sunday claimed “perfect success” in an underground test of what it called a hydrogen bomb.
It was the North’s sixth nuclear test since 2006.
US defence secretary Jim Mattis hit back, saying the US will answer any threat from the North with a “massive military response, a response both effective and overwhelming”.
Earlier, Mr Trump threatened to halt all trade with countries doing business with the North, a veiled warning to China, and faulted South Korea for its “talk of appeasement”.
The tough talk from America’s commander in chief and the retired general he picked to oversee the Pentagon came as the administration searched for a response to the escalating crisis.
Mr Trump, asked by a reporter during a trip to church if he would attack the North, said: “We’ll see.”
No US military action appeared imminent, and the immediate focus appeared to be on ratcheting up economic penalties, which have had little effect so far.
The UN Security Council scheduled an emergency meeting at the request of the US, Japan, France, Britain and South Korea.
It would 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.
Members of the US Congress expressed alarm at the North’s test and emphasised strengthening US missile defences. Leaders in Russia, China and Europe issued condemnations.
Mr Mattis told reporters America does not seek the “total annihilation” of the North, but added: “We have many options to do so.”
The administration has emphasised its pursuit of diplomatic solutions, knowing the potentially horrific costs of war with the North.
But the decision to have Mr Mattis deliver a public statement seemed to suggest an escalating crisis.
The precise strength of the underground nuclear explosion is not yet clear.
South Korea’s weather agency said the artificial earthquake caused by the explosion was five times to six times stronger than tremors generated by the North’s previous five tests.
North Korea’s state-run television broadcast a special bulletin to announce the test, and said Mr Kim attended a meeting of the ruling party’s presidium and signed the go-ahead order.
Earlier, the party’s newspaper published photos of him examining what it said was a nuclear warhead being fitted onto an intercontinental ballistic missile.
Sunday’s detonation builds on recent North Korean advances that include test launches in July of two ICBMs that are believed to be capable of reaching the mainland US.
The North says its missile development is part of a defensive effort to build a viable nuclear deterrent that can target US cities.
North Korea’s accelerating push to field a nuclear weapon that can target all of the US is creating political complications for Washington as it seeks to reassure allies it will uphold its decades-long commitment to deter nuclear attack on South Korea and Japan.