SEOUL, South Korea (AP) — The Latest on Secretary of State Rex Tillerson's visit to Asia (all times local):

Secretary of State Rex Tillerson in Washington on March 6 discussing issues related to visas and travel after President Donald Trump signed a new travel ban. REUTERS/Kevin Lamarque SEOUL, South Korea — US Secretary of State Rex Tillerson said Friday it may be necessary to take preemptive military action against North Korea if the threat from its weapons program were to reach a level "that we believe requires action."

Tillerson outlined a tougher strategy to confront North Korea's nuclear threat after visiting the world's most heavily armed border near the tense buffer zone between rivals North Korea and South Korea.

Asked about the possibility of using military force against the North, Tillerson told a news conference in the South Korean capital, "all of the options are on the table."

He said the US did not want a military conflict, "but obviously if North Korea takes actions that threatens South Korean forces or our own forces, that would be met with (an) appropriate response."

"If they elevate the threat of their weapons program to a level that we believe requires action," he said, "that option is on the table."

But he said that by taking other steps, including sanctions, the US was hopeful that North Korea could be persuaded to take a different course before it reaches that point.

Past US administrations have considered military force because of North Korea's development of nuclear weapons and ballistic missile to deliver them, but rarely has that option been expressed so explicitly.

North Korea has accelerated its weapons development, violating multiple UN Security Council resolutions and appearing undeterred by tough international sanctions. The North conducted two nuclear test explosions and 24 ballistic-missile tests last year. Experts say it could have a nuclear-tipped missile capable of reaching the US within a few years.

Tillerson is midway through a three-nation swing through Northeast Asia, which began in Japan and will end in China. State Department officials have described it as a "listening tour" as the administration seeks a coherent North Korea policy, well-coordinated with its Asian partners.

Earlier Friday, Tillerson touched down by helicopter Friday at Camp Bonifas, the US-led UN base about 438 yards from the Demilitarized Zone, a Cold War vestige created after the Korean War ended in 1953. He then moved to the truce village of Panmunjom inside the DMZ, a cluster of blue huts where the Korean War armistice was signed.

Tillerson is the latest in a parade of senior US officials to have their photos taken at the border. But it's the first trip by the new Trump administration's senior diplomat.

North Korean leader Kim Jong Un inspecting the construction site of Ryomyong Street in an undated photo released by North Korea's Korean Central News Agency. Reuters/KCNA

The DMZ, which is both a tourist trap and a potential flashpoint, is guarded on both sides with land mines, razor-wire fence, tank traps, and hundreds of thousands of combat-ready troops. More than a million mines are believed to be buried inside the DMZ. Land-mine explosions in 2015 that Seoul blamed on Pyongyang maimed two South Korean soldiers and led the rivals to threaten each other with attacks.

Hordes of tourists visit both sides, despite the lingering animosity. The Korean War ended with an armistice, not a peace treaty, which means the Korean Peninsula remains in a technical state of war.

President Donald Trump is seen as seeking to examine all options — including military ones — for halting the North's weapons programs before Pyongyang becomes capable of developing a nuclear-tipped missile that could reach the US mainland.

Central to the US review is China and its role in any bid to persuade Pyongyang to change course. China remains the North's most powerful ally.

While the US and its allies in Seoul and Tokyo implore Beijing to press its economic leverage over North Korea, the Chinese have emphasized their desire to relaunch diplomatic talks — a nonstarter for the US under current conditions.

The US and China also disagree over US deployment of a missile-defense system to South Korea. The US says it's a system focused on North Korea. China sees it as a threat to its own security.

Last week, North Korea launched four missiles into seas off Japan, in an apparent reaction to major annual military drills the US is conducting with South Korea. Pyongyang claims the drills are a rehearsal for invasion.

In Beijing, a North Korean diplomat said Thursday that Pyongyang must act in self-defense against the drills, which he said had brought the region to the brink of nuclear war. He said the drills were aimed at using atomic weapons for a preemptive strike against North Korea. Washington says the maneuvers are routine and defensive.

During last year's election campaign, Trump called into question US security alliances and called for Tokyo and Seoul to contribute more for their defense. Tillerson, however, stressed that cooperation with Japan and South Korea was "critical."

Japan and South Korea both host tens of thousands of US troops. Washington has been urging its two allies to step up security cooperation despite their historically strained relations.