Beijing objects to both the military drills and the deployment in South Korea of an advanced USA missile defence system that has a radar that can see deep into Chinese territory.
But Sunday's test, the sixth and most powerful North Korea has conducted since its first in 2006, was a stunning advance in its demonstrated ability to build high-yield nuclear weapons.
North Korea might launch an intercontinental ballistic missile (ICBM) on Saturday, as the country marks the anniversary of its founding, South Korean Prime Minister Lee Nak-yon on Thursday.
The U.S. defense chief assured that the two allies are "tied tightly together" and South Korea will not face a possible North Korean provocation alone. They'll be weak and they'll be more susceptible to North Korea exploiting that power.
Just ahead of the test, North Korea's state-run media released photos showing leader Kim Jong Un surrounded by the country's top nuclear scientists inspecting what it called a two-stage thermonuclear weapon.
His remarks were made on the sidelines of an economic forum in the Russian port city of Vladivostok which is also being attended by Russian President Vladimir Putin and South Korea's Moon Jae-In.
"There is very little chance that we are ever going to talk this guy out of his weapons, and none of us who have been watching the situation closely for years really thought we were going to", as Mira Rapp-Hooper, a scholar at Yale Law School who studies North Korea, put when I spoke to her this week.
Russian Federation has played a much more visible role in the North Korea crisis since the nuclear test.More news: Mobile Sets Unlimited Netflix Promotion Ahead Of Apple Launch
North Korea says it needs its weapons to protect itself from USA aggression. It is an extremely risky and uncertain calculation that sets the stage for further nuclear and missile tests.
The morning protests began as the U.S. threatened to impose sanctions on countries that trade with the North ahead of a crucial meeting of the United Nations security council, to discuss new measures against the regime.
Washington has presented a draft United Nations resolution calling for an oil embargo on North Korea, an assets freeze on Kim Jong-Un, a ban on textiles and an end to payments of North Korean guest workers.
In tweets earlier this week, Trump threatened to halt all trade with countries doing business with North Korea, a clear warning to China. Sunday's test of what North Korea said was an advanced hydrogen bomb was its largest by far.
South Korea can not accept war as an option to deal with the North Korean nuclear threat, a senior Seoul diplomat said Tuesday in the wake of Pyongyang's sixth and most powerful atomic test.
Overflights of the Korean Peninsula by heavy bombers such as the B-1B have incensed Pyongyang.
So, while China's leaders probably dislike Kim Jong-un as much as the Americans do and want an end to his reckless behavior, they are unlikely to heed Trump's calls to help him bring the tyrant to his knees, even if they could. "China has not just influence but has numerous levers that are needed to change behaviour in North Korea", Fallon told BBC radio.
Seoul has hardened its stance against Pyongyang after its torrent of arms tests, the latest on Sunday being a detonation of what North Korea said was a thermonuclear weapon built for missiles capable of reaching the United States mainland. "China will promote denuclearization and the maintenance of stability on the peninsula, and promote solving problems on the peninsula through dialogue and consultation".