Mattis along with Secretary of State Rex Tillerson held high-level security talks with their Chinese counterparts in Washington on Wednesday (21 June), where they pressed Beijing to exert more economic and diplomatic pressure on Pyongyang to prevent escalating tensions in the Korean peninsula.
Tillerson's remarks followed a meeting at the State Department with Mattis, Chinese State Councilor Yang Jiechi and General Fang Fenghui, chief of the People's Liberation Army's Joint Staff Department.
However, on Tuesday, Trump sent a tweet suggesting Xi's efforts had not borne fruit - a message he reiterated before supporters in Iowa.
While there is little doubt Xi Jinping's leadership will be extended by the party hierarchy in September, he is reluctant to get involved in foreign distractions, China expert Steven Lee Myers wrote in the New York Times Wednesday.
"In regards to the president's view of North Korea, I believe he represents the American people's view of North Korea right now", Mattis said of Mr. Trump.
Earlier this month, Mattis seemed satisfied with China's effort to mitigate the North Korean threat telling attendees of the annual Shangri-La Dialogue: "The Trump administration is encouraged by China's renewed commitment to work with the global community toward denuclearization".
Trump has not elaborated on what might happen next if China, by far the North's most important trading and diplomatic partner, is out of ideas.
U.S. and China both said in Wednesday's talks that their companies shouldn't be allowed to do business with North Korean entities covered under United Nations sanctions.
Trump has made halting the North Korean nuclear threat his No. 1 foreign policy priority.
On Wednesday the U.S. leader took the formal step of extending for another year a national emergency with respect to North Korea that was first decreed in 2008 under president George W. Bush.
"We had an open and frank dialog about what more can be done in areas of common interest", he said. He was addressing a gathering at an Iowa community college along with state Governor Terry Branstand, the new USA ambassador to Beijing. "We'll continue to work to close gaps in our understanding and to work in some kind of manner in the future that removes these irritants", Mattis said.
Washington has 28,000 troops deployed in South Korea and a naval armada in the region.
China accounts for 90 percent of world trade with North Korea.
More than 2,500 family, friends and well-wishers gathered for the funeral of Otto Warmbier, the United States student who died after falling into a coma while in prison in North Korea. His release last week initially seemed a gesture of goodwill by Pyongyang, but it quickly turned sour. "We see a young man go over there healthy and with a minor act of mischief" and come home on the verge of death.
"What you are seeing I think is the American people's frustration with the regime that provokes and provokes and provokes and basically (is) playing outside the rules, plays fast and loose with the truth, that sort of thing".