Hours after North Korea said it successfully set off a hydrogen bomb in its sixth nuclear test, President Trump took aim at the nation in a series of tweets, calling its words and actions "hostile and dangerous" to the United States.

He also tweeted about China and South Korea.

"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," Mr. Trump tweeted.

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"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!" he added.

North Korea has conducted a major Nuclear Test. Their words and actions continue to be very hostile and dangerous to the United States..... — Donald J. Trump (@realDonaldTrump) September 3, 2017

..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. — Donald J. Trump (@realDonaldTrump) September 3, 2017

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! — Donald J. Trump (@realDonaldTrump) September 3, 2017

Mr. Trump's apparent criticism of a diplomatic approach toward North Korea follows a Wednesday meeting in which U.S. Defense Secretary James Mattis met with his South Korean counterpart, emphasizing the notion that diplomatic means are still not off the table in its handling of the regime.

"We're never out of diplomatic solutions," Mattis said. "We always look for more. We're never complacent."

North Korea's state-run television broadcast a special bulletin Sunday afternoon to announce its latest test, claiming that it tested a thermonuclear weapon -- commonly called an H-bomb. That could be hard to independently confirm. It said the underground test site did not leak radioactive materials, which would make such a determination even harder.

At the same time, the simple power of the blast was convincing. Japan's Defense Minister Itsunori Onodera said it might have been as powerful as 70 kilotons. North Korea's previous largest was thought to be anywhere from 10 to 30 kilotons.

"We cannot deny it was an H-bomb test," Onodera said. "North Korea might have successfully tested a weapon with significantly large capability."

Though the precise strength of the blast has yet to be determined, South Korea's weather agency said the artificial earthquake it caused was five to six times stronger than tremors generated by its previous tests. It reportedly shook buildings in China and in Russia.

The test was carried out at 12:29 p.m. local time at the Punggye-ri site where North Korea has also conducted past nuclear tests. Officials in Seoul put the magnitude at 5.7, while the U.S. Geological Survey said it was a magnitude 6.3. The strongest artificial quake from previous tests was a magnitude 5.3.

The nuclear test is the North's first since Mr. Trump assumed office in January. Mr. Trump has been talking tough with the North over its stepped-up missile tests, including a comment last month that Pyongyang "will be met with fire and fury like the world has never seen" if it continued to threaten the United States.

North Korea conducted two nuclear tests last year, the last nearly a year ago, on the Sept. 9 anniversary of the nation's founding. It has been launching missiles at a record pace this year. It conducted its most provocative launch yet last month, in response to ongoing U.S.-South Korea military exercises, when it fired a potentially nuclear-capable midrange missile over northern Japan.

It said that launch was the "curtain-raiser" for more activity to come.

The North claims the device was made domestically and has explosive power that can range from tens to hundreds of kilotons. Outside experts suggested the yield of the device tested Sunday might be in that ballpark, though closer to the lower range. For context, the bomb dropped on Hiroshima by the United States had a 15-kiloton yield.

North Korea's nuclear and missile program has made huge strides since Kim Jong Un rose to power following his father's death in late 2011.

White House Press Secretary Sarah Huckabee Sanders issued a statement Sunday morning.

"The National security team is monitoring this closely," the statement said. "The President and his national security team will have a meeting to discuss further later today. We will provide updates as necessary."