By Ju-min Park and James Pearson
SEOUL (Reuters) - A U.S. policy of strategic patience with North Korea's nuclear and missile programs has ended, Secretary of State Rex Tillerson said in South Korea on Friday, warning that military action would be "on the table" if Pyongyang elevated the threat level.
On his first Asian visit as secretary of state, Tillerson will travel to China on Saturday with a main focus on finding a "new approach" on North Korea after what he described as two decades of failed efforts to denuclearize the insular nation.
"Let me be very clear: the policy of strategic patience has ended. We are exploring a new range of security and diplomatic measures. All options are on the table," Tillerson told a news conference in Seoul.
In spite of his 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.
U.S. President Donald Trump said on Friday that North Korea was "behaving very badly" and accused Pyongyang's neighbor and only ally 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!"
Tillerson, a former oil executive with no prior diplomatic experience, said any North Korean actions that threatened the South would be met with "an appropriate response".
"If they elevate the threat of their weapons programme to a level that we believe requires action, that option is on the table," he said when asked about military action.
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.
Tillerson is due to 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.
"We believe these actions are unnecessary and troubling," Tillerson said, referring to what South Korea sees as Chinese retaliation in the form of business restrictions in response to the deployment of the missile system.
"We also believe it is not the way for a regional power to help resolve what is a serious threat for everyone. So we hope China will alter its position on punishing South Korea."
"We hope they will work with us to eliminate the reason THAAD is required."
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.
FACING OFF AT THE DMZ
Earlier, Tillerson visited the Demilitarized Zone, and looked across the heavily fortified border at armed North Korean guards, staring back. He met some of the 28,500 U.S. troops stationed in South Korea.
Tillerson also met South Korean Prime Minister Hwang Kyo-ahn, who is also acting president.
The latest bout of tension with North Korea comes at a time of political turmoil in South Korea. President Park Geun-hye was ousted last week after being impeached in a corruption scandal and an election for a new president will be on May 9.
A liberal opposition politician, Moon Jae-in, who has raised questions about the THAAD deployment, is leading in the opinion polls.
Tillerson said he expected a new government would "continue to be supportive" of the deployment, adding it was also intended to protect U.S. troops in South Korea.
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 neighbour of the peninsula, China has even more reason than any other country to care about the situation," she told a briefing.
Hua also said the THAAD would "upset the regional strategic balance". Its radar, with a range of more than 2,000 km (1,250 miles), meant it could cover a large part of China, far outside the scope of the threat South Korea faces, Hua said.
"We do not oppose South Korean taking necessary measures to protect its security, but these measures cannot be based upon harming the security interests of South Korea's friendly neighbour, China," she said.
(Additional reporting by Ben Blanchard in BEIJING, Christine Kim in SEOUL and David Brunnstrom and Susan Heavey in Washington; Writing by Robert Birsel; Editing by Raju Gopalakrishnan and Alistair Bell)