SEOUL, South Korea — Secretary of State Rex Tillerson warned for the first time Friday that “all options” are being considered to counter North Korea’s emerging nuclear threat, including a militarily strike if necessary to safeguard allies and tens of thousands of U.S. troops stationed in the region.

The threat of a U.S. military attack comes after a series of ballistic missile tests by Kim Jong Un’s government in recent weeks have heightened tensions across northeast Asia and raised the possibility of a conflict with an adversary that now possesses nuclear arms and appears close to being able to strike U.S. territory.

The tough talk appears to be a break from previous U.S. administrations, which emphasized diplomacy, economic sanctions and covert operations, including cyberattacks, to try to reduce the danger from one of the world’s most isolated, and unpredictable, dictatorships.

“Certainly we do not want for things to get to a military conflict,” Tillerson told reporters in Seoul on the second leg of his three-nation visit to Asia, his first to the region since taking office.

“We’ve been quite clear on that in our communications. But obviously, if North Korea takes actions that threaten the South Korean forces or our own forces, then that will be met with an appropriate response,” he added.

“Let me very clear: The policy of strategic patience has ended,” he said, referring to the Obama administration’s policy of trying to wait out the North Korean regime while pressing it with economic sanctions and covert actions.

Tillerson arrives in Beijing on Saturday, where he is expected to meet President Xi Jinping. China remains North Korea’s chief political and economic patron but it has struggled over the years to rein in Pyongyang’s leadership.

Tillerson emphasized the need for maintaining economic sanctions on Pyongyang but also made clear that the Trump administration would not be limited to that approach.

“We’re exploring a new range of diplomatic, security and economic measures. All options are on the table,” he said.

Among the options would be to boost South Korea’s anti-missile defenses, a process that is underway, or to enable Japan to build an offensive missile capability. Japan’s 1947 Constitution, imposed by the United States, limits its military to defense only.

Washington also could reintroduce nuclear weapons to U.S. bases in South Korea to serve as a front-line deterrent. They were removed in 1991 under President George H.W. Bush as part of a post-Cold War effort to ease global nuclear tensions.

Previous administrations have considered a first strike against North Korean missile and nuclear facilities an option of last resort because it almost certainly would provoke a massive retaliation against South Korea and Japan. More than 75,000 U.S. military personnel are stationed in those countries.

The mounting threat could pose the first major foreign policy crisis for the Trump White House. As a candidate, Trump suggested letting Japan and South Korea build their own nuclear weapons to counter North Korea, but he has not pursued that as president.

Even the threat of a preemptive American attack adds risks to a volatile situation since North Korea has always insisted that was the U.S. intention. Its leaders have used that claim to justify creating one of the world’s most heavily armed states.

Tillerson also appeared to reject the idea of trying to negotiate a freeze in North Korea’s weapons program, a policy the Clinton administration tried in 1994 by supplying oil and other aid to Pyongyang in an effort to block its then-nascent nuclear development.

The so-called Agreed Framework successfully slowed Pyongyang’s ability to produce weapons-grade plutonium that could be used to fuel a bomb. But the deal collapsed in 2002 when Pyongyang shifted course and pursued a uranium-enrichment route to nuclear arms.

It conducted its first underground nuclear test in 2006 under the Bush administration, and four tests since then. The most recent, last September, was said to produce a destructive yield larger than the nuclear bomb that the U.S. dropped on Hiroshima in 1945 at the end of World War II.

Another attempted freeze, under the Obama administration in 2012, lasted only 15 days before Pyongyang launched a missile that the U.S. said violated the deal.

North Korea’s capabilities have expanded dramatically in the past year, suggesting it is getting closer to building an intercontinental ballistic missile that could carry a warhead to Alaska, Hawaii or the continental U.S.

It conducted more than two dozen missile tests, including one that could reach all of Japan, another launched from a submarine, and another that uses solid fuel and can be launched quickly.

An unusual launch this month involved four medium-range missiles. North Korea said the simultaneous salvo was a drill to practice hitting U.S. military bases in Japan. The missiles all fell in the ocean.

“At this stage, I’m not sure we would be willing to freeze with the circumstance where they exist today, given that would leave North Korea with significant capabilities that would represent a true threat not just to the region but to American forces as well,” Tillerson said.

On Thursday, the North Korean Embassy in Beijing invited reporters in for a rare news conference to blame the United States for putting the region at what it called “the brink of nuclear war.”

The bellicose language was not new but issuing the threat in Beijing, which the Trump administration hopes will help constrain Pyongyang, was notable. China has announced a ban on coal imports from North Korea, but analysts doubt Beijing will enforce the ban for fear of creating instability on its border.

For his part, President Donald Trump declared on Twitter that North Korea was “behaving very badly” and dismissed a Chinese proposal to freeze North Korea’s nuclear and missile programs in exchange for a halt to U.S.-South Korean military exercises.

Tillerson’s remarks, standing with South Korean Foreign Minister Yun Byung-se, came a day after he declared in Tokyo that two decades of attempts to block North Korea from developing nuclear weapons and ballistic missiles had failed and that a “different approach” was required.

Earlier Friday, Tillerson toured the Demilitarized Zone, a heavily guarded buffer area between North and South Korea created after the 1953 armistice that halted fighting during the Korean War. The two nations have never signed a formal peace treaty.

A group of North Koreans, apparently tourists, waved from across the border during Tillerson’s visit. A helmeted North Korean soldier, just across the border, took pictures of Tillerson’s back as he posed with Gen. Vincent K. Brooks, commander of U.S. forces in South Korea.

Tillerson ate lunch with U.S. troops there and signed a brick with chalk, a tradition for dignitaries who visit the site.

Tillerson’s tour of the region comes as the U.S. military is participating in a two-month exercise with South Korean and Japanese forces, an annual exercise that North Korea routinely denounces as a prelude to war.

The Foal Eagle exercise involves fighter jets, submarines and ground forces involved in a range of complex drills. About 3,600 U.S. service members were deployed for the event, joining the 28,000 U.S. troops permanently based in South Korea.

Capt. Jeff Davis, a Pentagon spokesman, told reporters Friday that the U.S. military has a number of plans in place in the event of hostile North Korean military action. He would not directly comment on the possibility of a preemptive U.S. attack.

“I’ll let Secretary Tillerson talk for U.S. policy,” Davis said. “Our job is to provide military options that give strength to foreign policy that he leads.”

Tillerson’s meeting in Seoul comes amid political upheaval in South Korea. Its president was removed from office last week in a corruption scandal that threatens nearly a decade of conservative party rule.

The leading candidate in the polls, Moon Jae-in, says he wants to delay installation of a new U.S. anti-missile system, known as THAAD, which is intended to shoot down North Korean missiles.

The U.S. began moving parts of the system into South Korea last month, although it is not yet operational. The missile battery is to be installed on Jeju Island, a popular destination for Chinese tourists in southern South Korea that’s about 300 miles from Shanghai.

Chinese officials have complained that the system’s sophisticated radar would undermine China’s own military deterrent, and have retaliated by disrupting tourism and commerce with South Korea.

“While we acknowledge China’s opposition, its economic retaliation is inappropriate and troubling,” Tillerson said, calling on China to end the practice.

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(Staff writer Wilkinson reported from Washington and special correspondent Stiles from Seoul. Special correspondent Jessica Meyers in Beijing and staff writer W.J. Hennigan in Washington contributed to this report.)