US President Donald Trump says North Korea is "behaving very badly" and China has done little to help, hours after Secretary of State Rex Tillerson said a US policy of strategic patience with North Korea had ended.
"North Korea is behaving very badly. They have been 'playing' the United States for years. China has done little to help!" Mr Trump said in a message on Twitter on Friday.
Mr Tillerson, who is currently in Seoul, is due to visit China on Saturday and Sunday to meet Chinese President Xi Jinping.
Mr Tillerson says it may be necessary to take pre-emptive military action against North Korea if the threat from its weapons programme reaches a level "that we believe requires action."
He spoke at a news conference in Seoul after visiting the heavily militarised border with North Korea on Friday.
Asked about the possibility of using military force against the North, Mr Tillerson said: "All of the options are on the table."
He said the US does not want a military conflict, but if North Korea takes actions that threaten South Korean or US forces it would be met with an "appropriate response."
Past US administrations have considered military force because of North Korea's development of nuclear weapons, but rarely has that option been expressed so explicitly.