SEOUL (Reuters) - U.S. Secretary of State Rex Tillerson on Friday issued the Trump administration’s starkest warning yet to North Korea, saying that a military response would be “on the table” if Pyongyang took action to threaten South Korean and U.S. forces.

Speaking in Seoul after visiting the Demilitarized Zone dividing the Korean peninsula and some of the 28,500 U.S. troops stationed in South Korea, Tillerson said former President Barack Obama’s policy of “strategic patience” towards Pyongyang’s nuclear and missile programs was over.

“We are exploring a new range of security and diplomatic measures. All options are on the table,” Tillerson told a news conference.

He said any North Korean actions that threatened U.S. or South Korean forces would be met with “an appropriate response,” turning up the volume of the tough language that has marked President Donald Trump’s approach to North Korea.

“Certainly, we do not want for things to get to a military conflict,” he said when asked about possible military action, but added: “If they elevate the threat of their weapons program to a level that we believe requires action, that option is on the table.”

In spite of Tillerson’s warning, U.S. officials have stressed that while an ongoing review on North Korea policy includes military options, such contingency planning has been conducted for decades and that the preferred course is to press Pyongyang to abandon its weapons programs through increased sanctions and other diplomatic pressure, particularly on China.

Tillerson, a former oil executive with no prior diplomatic experience, travels to China on Saturday, where he will press Beijing, North Korea’s only ally, to do more to rein in its neighbor.

The main focus of Tillerson’s trip, his first visit to Asia as secretary of state, has been on developing a “new approach” to North Korea after what he described as two decades of failed efforts to persuade it to denuclearize. Tillerson also visited Japan on his trip.

Trump said on Friday that North Korea was “behaving very badly” and accused China of doing little to resolve the crisis over the North’s weapons programs.

“They have been ‘playing’ the United States for years.” Trump said in a tweet, referring to North Korea. “China has done little to help!”

ACTION WOULD BE HIGHLY RISKY

For now, U.S. officials consider pre-emptive military action against North Korea far too risky, given the danger of igniting a regional war and causing massive casualties in Japan and South Korea and among tens of thousands of U.S. troops based in both allied countries.

Such ideas could gain traction, however, if North Korea proceeds with a threatened test of an intercontinental ballistic missile capable of hitting the United States. Just before he took office in January, Trump tweeted: “It won’t happen!” when Kim said North Korea was close to testing an ICBM.

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Any preemptive attack on North Korea carries huge risks.

“As a practical matter I don’t see the administration deciding to preemptively strike North Korea’s capabilities,” Asia expert and former White House official, Mike Green of Washington’s Center for Strategic and International Studies said this week.

“We wouldn’t get them all and there’s a risk North Korea would open fire with its hundreds of missiles and thousands of artillery tubes and its nuclear and chemical and biological weapons on Japan and Korea and even China.”

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Tillerson will meet Chinese President Xi Jinping at the weekend and press him to do more on North Korea.

He called on Beijing to implement sanctions against North Korea and said there was no need for China to punish South Korea for deploying an advanced U.S. anti-missile system aimed at defending against North Korea.

China says the Terminal High Altitude Area Defense (THAAD) system’s powerful radar is a threat to its security.

North Korea has conducted two nuclear tests and a series of missile launches since the beginning of last year.

Last week, it launched four more ballistic missiles and is working to develop nuclear-tipped missiles that can reach the United States.

South Korean Foreign Minister Yun Byung-se told a joint news conference the missile system was only intended to defend against North Korea, not any other country.

China resents U.S. pressure to do more on North Korea and says it is doing all it can but will not take steps to threatened the livelihoods of the North Korean people.

It has urged North Korea to stop its nuclear and missile tests and said South Korea and the United States should stop joint military exercises and seek talks instead.

Chinese Foreign Ministry spokeswoman Hua Chunying reiterated that talks were the best way to resolve the problems of the Korean peninsula.

“As a close neighbor of the peninsula, China has even more reason than any other country to care about the situation,” she told a briefing.