SEOUL, South Korea  — Secretary of State Rex Tillerson said 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 the rivals Koreas. He also closed the door on talks with Pyongyang unless it denuclearizes and gives up its weapons of mass destruction.

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

Trump weighed in on the matter Friday on Twitter: “North Korea is behaving very badly. They have been ‘playing’ the United States for years. China has done little to help!”

Tillerson said the U.S. does not want a military conflict, “but obviously if North Korea takes actions that threaten 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 that option is 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 of North Korea’s development of nuclear weapons and ballistic missiles 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 met Friday with his South Korean counterpart Yun Byung-se and its acting president, Hwang Kyo-ahn on the second leg of a three-nation trip 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.