WASHINGTON/SEOUL - President Donald Trump followed up his incendiary warning to North Korea against threatening the United States with a boast on Wednesday about the strength of the American nuclear arsenal, although he expressed hope it would not need to be used.Trump's Twitter messages about the nuclear arsenal came after North Korea said it was considering plans for a missile strike on the US Pacific territory of Guam. That in turn followed Trump's comments on Tuesday that any North Korean threat to the United States would be met with "fire and fury.""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 tweeted. "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!"The sharp increase in tensions between a country that has one of the world's largest nuclear arsenals and an aspiring nuclear power rattled financial markets and prompted US Secretary of State Rex Tillerson to try to play down the rhetoric.While Trump said the nuclear arsenal was more powerful than ever before, US officials say it takes decades to actually modernize nuclear weapons, a move already under way under President Barack Obama's administration, and there are treaties that regulate nuclear expansion. The Trump administration is still conducting a nuclear posture review.Shortly before Trump's remarks on the nuclear arsenal, Tillerson landed in Guam for a previously scheduled visit after telling reporters he did not believe there was an imminent threat from North Korea and that "Americans should sleep well at night."Tillerson said that with his "fire and fury" warning, the US president was trying to use the kind of language that would resonate with North Korean leader Kim Jong Un. North Korea regularly threatens to destroy the United States."What the president is doing is 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.Earlier on Wednesday, North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a US military base that includes a submarine squadron, an airbase and a Coast Guard group.The plan would be put into practice at any moment, once Kim Jong Un made a decision, a Korean People's Army spokesman said in a statement carried by state-run KCNA news agency.Guam Governor Eddie Calvo dismissed the threat and said the island was prepared for "any eventuality" with strategically placed defenses. He said he had been in touch with the White House and there was no change in the threat level."Guam is American soil ... We are not just a military installation," Calvo said in an online video message.North Korea, which is pursuing missile and nuclear weapons programs in defiance of U.N. Security Council resolutions, also accused the United States of devising a "preventive war" and said in another statement that any plans to execute this would be met with an "all-out war, wiping out all the strongholds of enemies, including the US mainland."Washington has warned it is ready to use force if needed to stop North Korea's ballistic missile and nuclear programs but that it prefers global diplomatic action, including sanctions. The U.N. Security Council unanimously imposed new sanctions on North Korea on Saturday."COMPLEX AND SENSITIVE"Trump issued his strongest warning yet for North Korea in comments to reporters in New Jersey on Tuesday, saying that "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."John McCain, like Trump a Republican and heads of the Armed Forces Committee in the US Senate, said on Tuesday night the president should tread cautiously. "You've got to be sure you can do what you say you’re going to do," he said in a radio interview.The top Democrat in the House of Representatives, Nancy Pelosi, called Trump's "fire and fury" comments "recklessly belligerent."China, North Korea's closest ally despite Beijing's anger at Pyongyang's missile and nuclear programs, described the situation as "complex and sensitive," and urged calm and a return to talks."China calls on all sides to uphold the main direction of a political resolution to the Korean peninsula nuclear issue, and avoid any words or actions that may intensify the problem and escalate the situation," the government said in a statement sent to Reuters, repeating its customary stance.Catherine Ray, a spokeswoman for the European Union's executive arm in Brussels, told a news briefing on Wednesday the situation in North Korea is of "great concern" and called on the North to "refrain from any further provocative action."North Korea has made no secret of its plans to develop a nuclear-tipped missile able to strike the United States and has ignored all calls to halt its weapons programs.Pyongyang says its intercontinental ballistic missiles (ICBMs) are a legitimate means of defense against perceived U.S. hostility, including joint military drills with South Korea.The standoff drove investors out of stocks on Wednesday and into the safety of the yen, Swiss franc, gold and government debt. South Korea's benchmark index and Japan's Nikkei both closed down more than 1 percent.South Korea and the United States remain technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.The South Korean capital, Seoul, is home to roughly 10 million people and within range of massed North Korean rockets and artillery, which would be impossible to destroy in a first U.S. strike.Tens of thousands of U.S. troops remain stationed in South Korea and in nearby Japan, the only country to have been attacked with nuclear weapons. Wednesday was the 72nd anniversary of the atomic bombing of the city of Nagasaki by the United States.Tension in the region has risen since North Korea carried out two nuclear bomb tests last year and two ICBM tests in July.Japanese fighters conducted joint air drills with U.S. supersonic bombers in Japanese skies close to the Korean peninsula on Tuesday, Japan's Air Self Defence Force said.On Monday, two U.S. B-1 bombers flew from Guam over the Korean peninsula as part of its "continuous bomber presence," a U.S. official said, in a sign of the island's strategic importance.GUAM HAS ANTI-MISSILE SYSTEMGuam, popular with Japanese and South Korean tourists, is protected by the advanced U.S. Terminal High Altitude Area Defense (THAAD) anti-missile system, deployed in South Korea.Madeleine Z. Bordallo, the US Congresswoman for Guam, said she was confident US forces could protect it from the "deeply troubling" North Korean nuclear threat. She called on Trump to show "steady leadership" and work with the international community to lower tension.North Korea meanwhile said on Wednesday it had released a Canadian pastor serving a life sentence there on humanitarian grounds.There was no obvious connection between this and the standoff with the United States, but North Korea has in the past attracted the attention of Washington, and visits by high-profile Americans, with the detention and release of U.S. citizens.