Secretary of State Rex Tillerson tried to ease concerns the U.S. was gearing for war with North Korea, a day after President Donald Trump rattled global markets with his warning that he could unleash “fire and fury” against Kim Jong Un’s regime.

“Americans should sleep well at night, have no concerns about this particular rhetoric of the last few days,” Tillerson told reporters on his plane after a stop in Southeast Asia. Trump “felt it necessary to issue a very strong statement directed at North Korea,” he said.

Trump’s threat reverberated around the world, sparking a sell-off and prompting a wave of criticism even from members of his own political party. Senator John McCain, a Republican, said he wasn’t sure Trump was ready to act, while Senator Ben Cardin, a Democrat from Maryland and ranking member on the Senate Foreign Relations committee, said Trump’s language was counterproductive.

The president returned to Twitter on Wednesday morning with a posting that said the U.S. nuclear arsenal “is now far stronger and more powerful than ever before.” He added in a follow-up message that “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!”

‘Wrong Direction’

“It’s not the way you should be conducting foreign policy,” Cardin said Wednesday on MSNBC’s “Morning Joe’’ program. “What the president is doing by making his own unilateral decisions that indicate that we’re ready to start a military confrontation -– that’s just moving in the wrong direction.’’

The Stoxx Europe 600 Index declined 1 percent as of 8:52 a.m. in New York, the largest drop in more than a week. Futures on the S&P 500 Index sank 0.3 percent, the largest decrease in almost five weeks.

Tillerson said the U.S. is engaged in a very active diplomatic effort to halt Kim’s pursuit of a nuclear weapon that could strike the U.S. mainland. He said that North Korea should be looking for talks “with the right expectation of what those talks will be about.”

Trump’s threat to hit North Korea came as that nation — reacting to new United Nations sanctions against its nuclear program — warned the U.S. would “pay dearly” and said it was examining plans to fire a missile toward an American military base on Guam. The exchange followed a Washington Post report, citing a Defense Intelligence Agency analysis, that Pyongyang successfully developed a nuclear warhead to use on its missiles.

“He has been very threatening, beyond a normal statement,” Trump said of Kim.

While global powers and financial markets have long been accustomed to over-the-top rhetoric from North Korea, the U.S. has traditionally taken a more diplomatic stance. Trump’s suggestion he might meet Kim’s threats with action startled markets and prompted a renewed focus on the narrowing list of options available.

What U.S.-North Korea Hostilities Might Look Like: QuickTake Q&A

Senator Lindsay Graham on Wednesday said Trump’s rhetoric created a “red line’’ that made it clear that the U.S. would be willing to take action if North Korea didn’t pull back.

“This is not a language problem. This is a North Korean regime trying to get the capability to strike America,’’ the South Carolina Republican said on “CBS This Morning.’’ “We’ve failed for 30 years. It’s time to try something new.”

Earlier in the day, Trump retweeted several reports from Fox News on the North Korea tensions, including this: “U.S. Air Force jets take off from Guam for training, ensuring they can ‘fight tonight’.”

Japan and South Korea, the two countries most at risk from a U.S. attack on North Korea, largely brushed off Trump’s threats. Yonhap News Agency cited an unidentified official at the presidential office in Seoul saying there’s no “imminent crisis.”

In a statement on Wednesday, China urged all sides to avoid escalating tensions and to return to dialogue.

North Korea’s reported progress on miniaturizing nuclear warheads — coupled with two test flights of intercontinental ballistic missiles in July — are raising pressure on Trump. Before taking office, he pledged to prevent North Korea from developing an ICBM: “It won’t happen,” he wrote on Twitter.

While Kim’s efforts to develop a missile capable of delivering a nuclear weapon to the continental U.S. face big technological hurdles, he has made significant progress. He still needs a rocket that can survive reentry and a guidance-and-control system capable of directing it to the U.S. without breaking up.