North Korea has accelerated its weapons development, violating multiple U.N. Security Council resolutions and appearing undeterred by tough international sanctions

Secretary of State Rex Tillerson said on Friday it may be necessary to take pre-emptive military action against North Korea if the threat from their weapons program reaches 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 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.”

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

Past U.S. administrations have considered military force because 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 U.N. 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 U.S. within a few years.

Tillerson is mid-way 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, U.S.-led U.N. base about 400 meters 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 U.S. officials to have their photos taken at the border. But it’s the first trip by the new Trump administration’s senior diplomat.

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.

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 U.S. mainland.

Central to the U.S. 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 U.S. 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 non-starter for the U.S. under current conditions.

The U.S. and China also disagree over U.S. deployment of a missile defense system to South Korea. The U.S. 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 U.S. is currently conducting with South Korea. Pyongyang claims the drills are a rehearsal for invasion.

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

During last year’s election campaign, presidential candidate Trump called into question U.S. 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 U.S. troops. Washington has been urging its two allies to step up security cooperation despite their historically strained relations.