Secretary of State Rex Tillerson says the U.S. doesn't want to take military action against North Korea, but "all of the options are on the table" if a serious threat arises. Tillerson made his frank remarks in a visit to South Korea on Friday, a day after saying diplomatic efforts "have failed" to persuade North Korea to abandon its nuclear program.Tillerson's Asia tour began in Japan and will end in China. The top American diplomat is traveling without a press contingent."The policy of strategic patience has ended," Tillerson said, according to The Korea Times, in an apparent reference to the Obama administration's approach. "We are exploring a new range of diplomatic, security, economic measures. All options are on the table."Discussing potential military action, Tillerson said that while the U.S. would prefer to avoid an armed conflict, "If they elevate the threat of their weapons program to a level that we believe requires action, then that option's on the table."North Korea is casting a long shadow over this trip, both for its potential nuclear threat and the recent flurry of missiles it tested, in a launch that sent weapons hundreds of miles toward Japan before landing in the Sea of Japan. In a news conference with Tillerson and Japanese Foreign Minister Fumio Kishida on Thursday, the Korean peninsula was mentioned more than 40 times.On Friday, concerns about North Korea prompted a missile evacuation drill on Japan's coast, where more than 100 residents of a town heard sirens and were told to take cover in an exercise that's seen as unprecedented.Tillerson will head to China this weekend, as NPR's Anthony Kuhn reports from Beijing: