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 a possibility if the threat posed by North Korea were to escalate.
"We are exploring a new range of diplomatic, security, economic measures. All options are on the table," Tillerson told reporters at a joint press conference with his South Korean counterpart Yun Byung-Se.
"Certainly we do not want for things to get to military conflict," he said. "If they elevate the threat of their weapons program to a level that we believe requires action, then, that option's on the table."
Tillerson announced the end of Washington's "strategic patience" -- the stance of the previous administration under Barack Obama.
The secretary of state is currently touring northeast Asia, after a missile launch last week that Pyongyang described as a drill for an attack on US bases in Japan.
Tillerson is expected in China on Saturday. Beijing is North Korea's main diplomatic ally and trade partner.
China shares US concerns over Pyongyang's attempts to build an arsenal of nuclear devices, but has also blamed Washington for escalating tensions.