US President Donald Trump threatened Sunday to cut off all trade with countries doing business with North Korea, an apparent warning to China following Pyongyang's sixth and most powerful nuclear test to date.

Trump tweeted the message ahead of a meeting with his national security team on what the North claimed to be an H-bomb explosion earlier in the day.

"The United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea," he said.

Such a ban would affect China the most as it is responsible for 90 percent of North Korea's trade.

In a series of tweets earlier in the day, Trump slammed the North and also accused South Korea of conducting a policy of "appeasement" toward the North.

"North Korea has conducted a major Nuclear Test," he said.

"Their words and actions continue to be very hostile and dangerous to the United States." Trump then added, "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."

Indicating a policy rift with Seoul, he went on to say, "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!"

Washington's first reaction came about eight hours after tremors were detected near the North's main nuclear test site in northeastern Punggye-ri. Pyongyang announced later that it had successfully tested an H-bomb which can be mounted on its intercontinental ballistic missiles.

Trump was later asked by a reporter whether he plans to attack the North.

"We'll see," he said.

US Treasury Secretary Steve Mnuchin, meanwhile, told Fox News that he would prepare new sanctions to cut off all North Korean trade with other countries.

He stopped short, however, of saying whether the sanctions would target businesses in China.

"If countries want to do business with the United States, they obviously will be working with our allies and others to cut off North Korea economically," Mnuchin said.

US Secretary of State Rex Tillerson said Washington would push for the "strongest sanctions" as he spoke by phone with his South Korean counterpart, Kang Kyung-wha, according to the South's foreign ministry.

Earlier, the top national security advisers to Trump and South Korean President Moon Jae-in discussed the test by phone, according to Moon's office. The allies' top military officers also held separate talks in a phone call and agreed on the need for "effective military responses," the South Korean joint chiefs of staff said.

The provocation sharply escalated tensions that were already at a new high following Pyongyang's two ICBM tests in July.

Last week, it test-fired an intermediate-range ballistic missile over Japanese territory in what analysts said was a demonstration of its capability to strike the US island of Guam in the western Pacific.

North Korea had earlier threatened to lob missiles toward the island, following Trump's threat to unleash "fire and fury" on the regime. (Yonhap)