Mattis and Secretary of State Rex Tillerson are hosting their Chinese counterparts on Wednesday for the US-China Diplomatic and Security Dialogue in Washington.
Brian Peterson, president of the National Association of Medical Examiners, said that it's clear that Warmbier suffered brain damage at some point.
"And we've had I think positive movement on China over the past five months of this administration".
While Trump has heaped praise on Xi for trying to contain North Korea, which counts on China for some 90 percent of its trade, the effort has delivered few results.
No cause of death has been determined for Warmbier, who was detained for almost a year-and-a-half in North Korea before being sent home in a coma last week.
President Donald Trump said on Monday that a "lot of bad things happened" to Mr Warmbier at the hands of the "brutal regime". FILE- In this June 15, 2017, file photo, Fred Warmbier, father of Otto Warmbier, a University of Virginia undergraduate student who was imprisoned in North Korea in March 2016, speaks during a news conference at Wyoming.
The decision means that, for now, Warmbier's official cause of death will remain unknown. About 17 United States citizens have been detained in the past 10 years, including the three Americans who remain imprisoned there.
Last month Beijing and Washington signed a limited deal to open new markets for each other's exports, and a long-standing friend of the Chinese leadership, Iowa Governor Terry Branstad, was confirmed as ambassador.
Schiff said a new law was important to show Congress' unity on North Korea, arguing that financial measures through the Treasury Department might be more effective than a passport ban because it would deter travel companies ferrying Americans. The problem here being that for any definite answer to be drawn, investigators would have to be in possession of medical history and documented history of treatment from North Korea itself, which is (obviously) not very forthcoming.
In meetings at the State Department involving USA and Chinese diplomats and defense chiefs, North Korea was to get "top billing", according to Susan Thornton, the senior US diplomat for East Asia.
A funeral for Otto Warmbier, seen here in North Korea in 2016, will be held Thursday at Wyoming High School, where he was an athlete and salutatorian of his 2013 class. Warmbier was released from detention in North Korea in a coma June 13 and died in a US hospital on June 19.
Senator McCain also slammed North Korea saying, "Let us state the facts plainly: Otto Warmbier, an American citizen, was murdered by the Kim Jong-un regime". Corker said. "That's something we're looking at".
Spicer said Trump would be willing to meet Kim under the right conditions, but "clearly we're moving further away, not closer to those conditions".
Trump's administration talked up the threat of military action earlier this year, sending aircraft carrier strike groups to the region.
Last month, California Democratic Rep. Adam Schiff and his Republican colleague Rep. Joe Wilson of SC introduced the North Korea Travel Control Act.
"If anything, this has even strengthened our argument", said the diplomat, who spoke on the condition of anonymity to address a US policy matter.
They were joined by Japanese and Korean F-15s performing two separate bilateral missions, the official said.
Pyongyang has defied sanctions and worldwide pressure, saying it will continue to strengthen its military capability to protect itself from the threat posed by the presence of United States forces in the region.