U.S. Secretary of State Rex Tillerson warned Friday that America’s policy of “strategic patience” with North Korea is over, and said pre-emptive military action is “on the table” if the rogue regime presses ahead with its nuclear weapons program.

Tillerson’s comments, delivered during his visit to Seoul, are the bluntest yet from the Trump administration amid a spiraling crisis over Pyongyang’s nuclear and missile programs, and signal a more aggressive U.S. response to the historically erratic Hermit Kingdom.

“Let me be very clear: The policy of strategic patience has ended,” Tillerson said at a joint press conference with South Korean Foreign Minister Yun Byung-se.

“Talk is not going to change the situation.”

The former ExxonMobil CEO said that after two decades of failed and costly attempts to contain Pyongyang’s nuclear development, it was time for a new approach.

“We are exploring a new range of security and diplomatic measures,” he said, referring to the possibility of a pre-emptive strike but offering no further elaboration. “If they elevate the threat of their weapons program to a level that we believe requires action, that option is on the table.”

Shortly after Tillerson delivered his comments in Seoul, President Trump weighed in on Twitter.

Leonid Petrov, a Korea specialist at Australian National University, told VICE News that the U.S. pivot to abandoning strategic patience for “strategic impatience” was a risky act of brinkmanship that, one which puts South Korea at greater risk of aggression,. But Daniel Pinkston, an international relations expert at South Korea’s Troy University, told VICE News that the remarks were unlikely to do much to change the status quo.

“Talk is not going to change the situation,” he said, adding that the he believed the tougher tone was largely for the benefit of the U.S. domestic audience.

Tillerson, who is currently on a three-nation tour of East Asia, cut his meetings with South Korean officials short, according to officials in Seoul, who said the U.S. Secretary was ‘fatigued.’

Tillerson’s busy diplomatic weekend comes amid rising tensions in the region over Pyongyang’s nuclear ambitions, as it rapidly pushes ahead with plans to develop a nuclear-tipped missile capable of reaching the U.S. Since the beginning of last year, North Korea has conducted two nuclear tests and a string of missile launches, including four fired toward Japan last week, in defiance of U.N. restrictions.

The U.S. deployment on South Korean soil of a sophisticated anti-missile system, the Terminal High Altitude Area Defense (THAAD), intended to defend against North Korean aggression, has further inflamed regional tensions and angered China, which views the system as a threat.

Ahead of a critical visit to China on Saturday, in which Tillerson will attempt to enlist Beijing’s help in reining in North Korea, he called on the Chinese leadership to drop its opposition to the anti-missile system and help “address the threat that makes THAAD necessary.” He also called China’s economic retaliation against South Korea for its involvement in the project “inappropriate and troubling,” and dismissed Beijing’s suggestion that the U.S. suspend military exercises with South Korea to defuse tensions.