US President Donald Trump says China has failed to persuade North Korea to rein in its nuclear program, ratcheting up the rhetoric after an American student detained by Pyongyang died days after his release.
Trump has been seeking greater cooperation from China to put pressure on its ally North Korea.
Trump and his Chinese counterpart Xi Jinping had a high-profile summit in Florida in April, where Trump said he hoped Xi would help resolve the North Korean standoff.
"While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!" Trump wrote in a tweet.
It was unclear whether Trump's remark represented a shift in US policy toward China or he was simply expressing frustration with North Korea.
"I think the president is signaling some frustration," former US ambassador to South Korea Christopher Hill told MSNBC. "He’s signaling to others that he understands this isn’t working, and he’s trying to defend himself, or justify himself, by saying that at least they tried as opposed to others who didn’t even try."
A US official said Tuesday that US spy satellites had picked up movements at North Korea's nuclear test site near a tunnel entrance.
It was not known if the movements were preparations for a new nuclear test - perhaps to coincide with high-level talks between US and Chinese officials in Washington on Wednesday.
Secretary of State Rex Tillerson and Defense Secretary James Mattis will meet with China's State Councilor Yang Jiechi, and General Fang Fenghui, chief of joint staff of the People’s Liberation Army, for diplomatic and security dialogue.
Officials and experts have warned for months that Pyongyang could carry out a sixth nuclear test at any time.
North Korea says it will not give up on its nuclear deterrence unless Washington ends its hostile policy toward Pyongyang and dissolves the US-led UN command in South Korea.
On Tuesday, the US State Department urged North Korea to release “as soon as possible” three Americans who are still detained by Pyongyang, following the death of Otto Warmbier.
"We hold North Korea accountable for Otto Warmbier's unjust imprisonment," department spokeswoman Heather Nauert said at a news briefing.
Warmbier was released on June 13 in a state of coma after Joseph Yun, the State Department’s special envoy on North Korea, paid a visit to Pyongyang and demanded his release.
Trump called Warmbier’s death a "total disgrace," adding that the college student might have been alive if he had been brought home sooner.