(CNN) US Secretary of State Rex Tillerson has sought to allay fears of a military confrontation with North Korea after President Donald Trump warned he could unleash "fire and fury" on the pariah state.

Tillerson defended Trump's comments but said there was no sign that the threat level from North Korea had changed and that Americans should "sleep well at night."

His unscheduled remarks, on a flight out of the region early Wednesday, appeared designed to dial back the unprecedented rhetoric from Trump.

In comments that were significantly more incendiary than any made by US presidents in the past, Trump had appeared to threaten nuclear war with North Korea.

"North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen," Trump said on Tuesday at his golf resort in Bedminster, New Jersey.

Hours after Trump spoke, and apparently in response to military exercises by the US earlier in the week, Pyonyang warned that it would carry out preemptive military strikes against the US, including the Pacific territory of Guam.

In a report on state-run media, North Korea said it would "turn the US mainland into the theater of a nuclear war" at the first sign of an impending American attack.

Tillerson was at pains to stress that the US did not believe that the threat from North Korea had changed."Nothing that I have seen and nothing that I know of would indicate that the situation has dramatically changed in the last 24 hours," he said, adding that "Americans should sleep well at night."

But he also defended Trump's comments."I think what the President was doing was sending a strong message to North Korea in language that Kim Jong Un would understand, because he doesn't seem to understand diplomatic language," Tillerson said.

Soon after he spoke, however, Trump touted US nuclear capabilities, potentially further escalating the standoff with Pyongyang. "My first order as President was to renovate and modernize our nuclear arsenal. It is now far stronger and more powerful than ever before," Trump wrote on Twitter just before 8 a.m. ET from his golf resort in New Jersey.

But in a second tweet, he seemed to play down the idea that the US would need to use them any time soon. "Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!"

Threats from North Korea

KCNA, North Korea's state media, said that a strike on Guam would be aimed at the US military bases on the Pacific island.

Guam's governor, Eddie Baza Calvo, released a video address Wednesday, reassuring the island's residents that there was no escalated threat. Guam's Homeland Security Advisor George Charfauros told CNN that he remains confident of the island's defenses.

But North Korea warned in a separate KCNA report on Wednesday that it was looking beyond Guam and would hit the US mainland with preemptive strikes, with the use of nuclear weapons, should there be any sign the US planned to strike North Korea first.

"The US should (remember), however, that once there observed a sign of action for 'preventive war' from the US, the army of the DPRK will turn the US mainland into the theater of a nuclear war before the inviolable land of the DPRK turns into the one," the report said.

White House sends mixed messages

Critics have said the White house has sent conflicting messages on its stance on North Korea, and Trump's aggressive approach is at odds with Tillerson's more diplomatic stance.

The threat from North Korea's nuclear and missile programs has been a top foreign policy priority for Trump since taking office in January, but the dangers posed by North Korea have taken center stage since the country test-fired two intercontinental ballistic missiles last month.

The latest crisis intensified when reports emerged this week that US intelligence sources believed North Korea had developed the ability to miniaturize a nuclear warhead to fit top of a missile. Those reports appeared to prompt Trump's tirade from Bedminster Tuesday.

Republican Sen. John McCain, chairman of the Senate Armed Services Committee, said Trump risked going too far.

"I take exception to the President's comments because you've got to be sure that you can do what you say you're going to do. In other words, the old walk softly but carry a big stick," McCain told KTAR radio in Arizona.

Democrats slammed Trump, saying his comments were "unhinged."

"President Trump is not helping the situation with his bombastic comments," California Sen. Dianne Feinstein said in a statement.

Tillerson has maintained that the US is open to dialogue with North Korea, if it promises to abandon its development of nuclear weapons.

But the US military has flexed its muscles by conducting joint military drills with Japan and South Korea and conducting show-of-force operations.

The Chinese Foreign Ministry on Wednesday warned against making aggravating remarks.

"China calls on the relevant sides to follow the broad direction of resolving the nuclear issue through political means, avoid remarks and actions that could aggravate conflicts and escalate tensions, and make a greater effort to return to the correct path of resolving the issue through dialogue and negotiations," it said in a statement.

China voted to move new sanctions against North Korea this week after the country test-fired two intercontinental ballistic missiles last month.

But Washington has pressed Beijing for more action on North Korea. China has leverage over the country as its biggest trading partner and main source of food and energy.

Pyongyang capabilities unchanged

Despite the threats little has changed in the assessment about Pyongyang's military capabilities and the chances of a US strike.

While US intelligence analysts have claimed that Pyongyang has produced a miniaturized nuclear warhead, it's not believed that the capability has been tested, according to the sources.

However, there's debate within the intelligence community that Pyongyang has the required skill and technology. The Washington Post, which was first to publish details, reported that it was the analysis of the Defense Intelligence Agency.

The threat from North Korea's nuclear and missile programs has been a top foreign policy priority for Trump since taking office in January, but the dangers posed by North Korea have taken center stage since the country was able to fire two intercontinental ballistic missiles.

Weapons experts say both of those missiles, designed to carry nuclear warheads, could theoretically reach the US mainland, based on the range of two recent missile tests.

North Korea observers have long maintained that a war between the US and North Korea is unlikely, largely for two reasons. The first being that both sides recognize how devastating another Korean War would be, the second being that the Kim regime, which values its survival above all else, knows it would lose.

Experts worry Trump's rhetoric could hurt the US by feeding North Korean insecurities and adding instability to an already tenuous situation.

"We have two inexperienced, impulsive presidents in control of these massive military machines," Joe Cirincione, president of Ploughshares Fund, a global security foundation, told CNN on Monday.

"It's one thing to make a mistake intentionally, its another thing to stumble into a conflict ... either one -- Kim Jong Un or Donald Trump -- could miscalculate and let loose a war unlike anything we have seen since World War II."