Defense Secretary Jim Mattis warned Monday that an attack by North Korea against the United States could quickly escalate into war, as he and Secretary of State Rex Tillerson moved to dial down tensions.

North Korea last week threatened to test-fire four missiles that would fly toward the small US Pacific island territory of Guam, following a promise from President Donald Trump to meet ongoing threats from Pyongyang with "fire and fury."

"If they fire at the United States, it could escalate into war very quickly," Mattis told Pentagon reporters.

Mattis's comments came after he and Secretary of State Rex Tillerson said in an opinion piece in The Wall Street Journal that America has "no interest" in regime change in Pyongyang or the accelerated reunification of the two Koreas, and stressed the importance of a diplomatic solution to the crisis.

The two statesmen decided to write the piece a couple of weeks ago, and it had not been in response to last week's heated rhetoric, Mattis said.

"We were thinking it would be wise to put out something that shows how the State Department and the Defense Department work together, it's not one or the other, it's the two working together," he said.

Mattis and Tillerson said the United States favors a diplomatic solution to the nuclear stand-off, especially with help from China, though they stressed diplomacy is "backed by military options."

"The US has no interest in regime change or accelerated reunification of Korea," Mattis and Tillerson wrote.

"We do not seek an excuse to garrison US troops north of the Demilitarized Zone. We have no desire to inflict harm on the long-suffering North Korean people, who are distinct from the hostile regime in Pyongyang."

Mattis and Tillerson called on China, which is North Korea's main trading partner and ally, to take advantage of an "unparalleled opportunity" to assert its influence on Pyongyang.

"If China wishes to play a more active role in securing regional peace and stability -- from which all of us, especially China, derive such great benefit -- it must make the decision to exercise its decisive diplomatic and economic leverage over North Korea," the US officials wrote.

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Mattis and Tillerson also credited UN efforts and said the United States was willing to negotiate with North Korea, but said Pyongyang should indicate a desire to show good faith by halting weapons and nuclear tests.

General Joseph Dunford, who chairs the US Joint Chiefs of Staff and is the nation's top uniformed military officer, met Monday with South Korea's defense minister Song Young-Moo and defense chief General Lee Sun-Jin.

He "conveyed America's readiness to use the full range of military capabilities to defend our allies and the US homeland," said spokesman Captain Darryn James.