WASHINGTON -- US President Donald Trump boasted about his country's nuclear arsenal Wednesday as tensions escalated over North Korea's nuclear and missile programs.

Trump wrote in an early morning tweet: "My first order as President was to renovate and modernize our nuclear arsenal. It is now far stronger and more powerful than ever before.

"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!"

A day earlier, he vowed to meet North Korea's threats to strike the US with "fire and fury."

North Korea hit back hours later, saying it is ready to carry out an attack near the US island of Guam.

The White House later explained that the president was referring to his order in January to conduct a Nuclear Posture Review to ensure the US nuclear deterrent is "modern, robust, flexible, resilient, ready, and appropriately tailored to deter 21st-century threats and reassure our allies."

En route home from a trip to Asia, Secretary of State Rex Tillerson assured Americans not to worry.

"I think Americans should sleep well at night, have no concerns about this particular rhetoric of the last few days," he told reporters on a flight to Guam, where he was to refuel, according to a transcript released by his department.

"I think that the president, again, as commander-in-chief, I think he felt it necessary to issue a very strong statement directly to North Korea," he added. "But I think what the president was just reaffirming is the United States has the capability to fully defend itself with any attack, will defend our allies, and we will do so."

Sebastian Gorka, deputy assistant to Trump, interpreted the president's latest tweet as a warning to Pyongyang.

"He's saying, 'Don't test America and don't test Donald J.

Trump,'" Gorka said in a Fox interview. "We are not just a superpower. We were a superpower. We are now a hyper power. Nobody in the world, especially not North Korea, comes close to challenging our military capabilities." (Yonhap)