"To resolve the Korean peninsula nuclear issue, China has been making unremitting efforts and we have been playing an important and constructive role", said Geng Shuang, a foreign ministry spokesman, while stressing China was not the "focus and the crux" of the crisis.
Trump had made clear earlier that his preference was to work with China on North Korea, but that he was willing to go after them alone.
"To see a young man go over there healthy and, (after) a minor act of mischief, come home dead basically... this goes beyond any kind of understanding of law and order, of humanity, of responsibility towards any human being", Mattis said.
"No conclusions about the cause and manner of Mr. Warmbier's death have been drawn at this time as there are additional medical records and imaging to review and people to interview", the coroner's office said in its statement.
Officials with the Hamilton County Coroner's Office announced Tuesday morning they were investigating Warmbier's death.
Ria Westergaard Pedersen, 33, who was with Warmbier in North Korea, told the Danish broadcaster TV2 that he had been nervous when taking pictures of soldiers, and said she doubted North Korea's explanation for his arrest.
Tillerson and Mattis were hosting Chinese State Councilor Yang Jiechi and General Fang Fenghui, the chief of joint staff of the People's Liberation Army. The meeting of top U.S. and Chinese diplomats and defense chiefs was held a day after President Donald Trump said China's efforts to use its leverage with Pyongyang had failed, raising fresh doubts about his administration's strategy for countering the threat from North Korea.
Thornton said talks would cover the South China Sea, where Beijing's island-building and construction of possible military facilities have rattled neighbors and caused tension with Washington; U.S.
Yun is a USA special envoy.
It was not immediately clear if Trump's tweet indicated a policy shift toward either North Korea or China, a longtime ally of the isolated state and its largest trading partner. He was held for almost a year-and-a-half in North Korea before being sent home in a coma last week.
He added that the two sides agreed they will not engage in business with North Korean organizations that are subject to United Nations sanctions.
US President Donald Trump has admitted that China has not succeeded in getting North Korea to curb its nuclear and ballistic missile programmes. "The North Koreans need to be held to account for that".