North Korea is preparing to launch a ballistic missile, possibly an ICBM, South Korean military sources have said.

Seoul's defence ministry also measured North Korea's nuclear test at 50 kilotons, Yonhap news agency reported.

The detonation was the strongest ever from the North, which claimed the test was of a hydrogen bomb.

South Korea's defence ministry said in a parliament hearing it was still seeing signs North Korea planned to stage more ballistic missile launches, possibly including an intercontinental ballistic missile.

"We have continued to see signs of possibly more ballistic missile launches. We also forecast North Korea could fire an intercontinental ballistic missile," said Chang Kyung-soo, a defence ministry official.

US: North Korea could be met with 'massive military response'

The defence ministry was called by parliament to answer questions about North Korea's sixth and largest nuclear test that was carried out a day earlier.

Following US warnings to North Korea of a "massive military response," South Korea's military fired missiles into the sea to simulate an attack on the North's main nuclear test site.

In a series of tweets, US President Donald Trump threatened to halt all trade with countries doing business with the North, a veiled warning to China, and faulted South Korea for what he called "talk of appeasement."

Mr Trump, asked by a reporter during a trip to church if he would attack the North, said: "We'll see."

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

South Korea'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.