SEOUL: US President Donald Trump on Friday accused China of not doing enough to help contain North Korea's nuclear ambitions, after his Secretary of State Rex Tillerson said military action against Pyongyang was an option.
"North Korea is behaving very badly. They have been 'playing' the United States for years. China has done little to help!" Trump said on Twitter.
Earlier in Seoul, Tillerson said military action would be an "option on the table" if the threat posed by North Korea were to escalate.
In strong statements that appear to signal a sea change in American policy towards the isolated country, the United States' top diplomat said North Korea's burgeoning missile and nuclear programmes must be halted.
"Certainly we do not want to, for things to get to military conflict," he told reporters in Seoul, but added: "If they elevate the threat of their weapons programme to a level that we believe requires action, then that option's on the table."
"The policy of strategic patience has ended," Tillerson told a joint press conference with his South Korean counterpart Yun Byung-Se.
"We are exploring a new range of diplomatic, security, economic measures. All options are on the table."
"Strategic patience" is the term given to the US policy under former President Barack Obama when the United States ruled out engaging the North until it made a tangible commitment to denuclearisation, hoping that internal stresses would bring about change.
Tillerson's visit to Asia -- South Korea is the second leg of the tour -- is his first foray into crisis management.
His remarks on Friday came a day after he said in Tokyo that 20 years of efforts to denuclearise the North had "failed" and promised a new approach.
North Korea has a long-standing ambition to become a nuclear power and conducted its first underground atomic test in 2006, in the teeth of global opposition.
Four more test blasts have followed, two of them last year.
Allowing the North to retain its present level of weapons technology was not appropriate, Tillerson said in Seoul.
"That would leave North Korea with significant capabilities that would represent a true threat."
The United Nations has imposed multiple sets of sanctions on Pyongyang over its nuclear and missile programmes, but its main diplomatic protector and trade partner China is accused of not fully enforcing them.
Tillerson will be going on to Beijing on Saturday to press it to do more.
"I don't believe we have ever fully achieved the maximum level of action that can be taken under the UN Security Council resolution with full participation of all countries.
"We know that other nations can take actions."
Earlier in the day, Tillerson had visited the Demilitarised Zone dividing the two Koreas to gaze on the North for himself.
Under the glaring eyes of alert North Korean soldiers, Tillerson toured the Panmunjom joint security area, guarded by both North Korea and the US-led United Nations Command since the Korean War ended with an armistice in 1953.
North Korean soldiers watched from their side of the demarcation line -- marked by cement blocks on the ground. At one point they were only a few feet from Tillerson, with one taking either video or photos.