BEIJING — Secretary of State Rex W. Tillerson and President Xi Jinping of China cast aside their differences on Sunday with a public display of cooperation, with both sides refraining from comment even as North Korea made another defiant statement by showing off a new missile engine.
In the highest-level face-to-face meeting between the two countries since Donald J. Trump became president, the two made no mention of other contentious issues, including possible punitive trade measures against China and Washington’s unhappiness with Beijing’s assertiveness in the South China Sea.
Mr. Xi, greeting the new secretary of state in an ornate room in the Great Hall of the People, thanked Mr. Tillerson for a smooth transition to the Trump administration and expressed his appreciation for the sentiment that “the China-U.S. relationship can only be defined by cooperation and friendship.”
At least in public, Mr. Tillerson adopted a far different tone than that of his boss, who said in a post on Twitter on Friday that China had “done little to help” on North Korea, instead saying that the United States looked forward to stronger ties with China.
China has been North Korea’s biggest backer, but relations between the two countries have been strained as the North continues to pursue the development of nuclear weapons. Hours before the meeting between Mr. Tillerson and Mr. Xi, North Korea stuck its nose under the tent, announcing that it had tested a new high-thrust missile engine that analysts said could be used in an intercontinental missile.
The test, apparently timed for Mr. Tillerson’s visit to Beijing, was another sign that North Korea was expanding its missile capabilities, with state media reporting that the country’s leader, Kim Jong-un, had presided over an event of “historic significance.”
During his 24-hour stay in Beijing, Mr. Tillerson, who also visited Japan and South Korea in his first trip to Asia as secretary of state, took the unusual step of echoing rosy Chinese language on the state of relations between the United States and China.
The relationship between China and the United States was guided by “nonconflict, nonconfrontation, mutual respect and win-win cooperation,” Mr. Tillerson said at a news conference with Foreign Minister Wang Yi.
But behind the scenes, diplomats and analysts said there was little doubt that Mr. Tillerson had pressed China to enforce sanctions against North Korea and had raised the possibility that the United States would bolster its missile defense in Asia if China did not rein in Mr. Kim.
China strongly objects to the installation in South Korea of a missile defense system there, and the polite public words from Mr. Tillerson were designed to give China “face,” said a diplomat in Beijing who spoke on the condition of anonymity per usual diplomatic custom.
Mr. Tillerson was almost certainly sterner in private, according to the diplomat. “I believe Tillerson repeated in the meetings what he said publicly in South Korea and Japan, and backed up Trump in his tweet,” the diplomat said.
That meant some public warmth was necessary, he said, because aside from talking about North Korea, Mr. Tillerson also had the task of setting a broad agenda for a summit meeting between Mr. Trump and Mr. Xi that is expected to take place in Florida in early April.
At the summit meeting, China is expected to seek a reaffirmation of the “One China” policy under which the United States recognizes a single government in Beijing and does not maintain diplomatic ties with Taiwan.
Mr. Trump committed to that policy in a telephone conversation with Mr. Xi in early February, but Chinese leaders, on edge about the president’s unpredictability, are anxious to nail that down even more clearly. Mr. Trump’s trade team is expected to be in place by the time Mr. Xi reaches Florida, and the Chinese will be eager to deter plans for tariffs and moves toward more stringent scrutiny of Chinese investment in the United States.
Chinese analysts said the secretary probably encountered resistance to his arguments that the missile defense system, known as Terminal High Altitude Area Defense, or Thaad, was of little danger to China, which firmly believes the system erodes its nuclear deterrent.
“Tillerson will repeat many times this is no threat to China, but Xi won’t believe it,” said Shi Yinhong, a professor of international relations at Renmin University.
The best chance for cooperation on North Korea might come with China deciding to more dutifully enforce some economic sanctions, Mr. Shi said.
That would be a relatively small price to pay the Americans for a smooth summit meeting in Florida, although it would further hurt China’s already strained ties with North Korea, he said.
“Maybe Xi will broaden the punishment against North Korea somewhat, at the cost of further damaging relations with North Korea,” Mr. Shi said. “We have punished North Korea many times, and Kim Jong-un hates China more and more. Maybe China will take some small steps to shut down a few trading companies, but not all.”
China keeps the rudimentary North Korean economy running by supplying almost all its oil, and there was little chance Mr. Xi would consider shutting down the pipeline, even though China abruptly halted imports of North Korea’s coal last month, ending a valuable source of foreign currency for the leadership in Pyongyang.
“China won’t turn the sanctions from targeting the North Korean nuclear program into a punishment for ordinary North Korean people,” The Global Times, a state-run newspaper that often reflects official thinking, said on Friday.
But on the eve of Mr. Tillerson’s visit to Beijing, a Washington-based research organization specializing in nuclear matters released a study that it said showed China was not enforcing the sanctions aimed at the nuclear program.
China had allowed large quantities of materials used to make a component of hydrogen bombs to pass through its borders to the North, according to the research group, the Institute for Science and International Security.
A newly operating plant in North Korea that produced a key ingredient for hydrogen bombs was a glaring example of China ignoring sanctions, the group said.
The study found that a plant producing lithium 6 — used in the manufacture of hydrogen bombs that are more powerful than conventional nuclear weapons — was located at a chemical complex on the North’s east coast.
North Korea had purchased mercury and lithium hydroxide in China, and the items had been transported across the border, the president of the institute, David Albright, said. The two commodities are needed for the production of lithium 6, he said.
A recent report by a United Nations Security Council panel of experts on how North Korea skirts sanctions said that the government was attempting to sell lithium 6, an indication that the country was not just producing the material but making more than it needed.
“Until today, North Korea has been able to advance its nuclear programs by buying in China, having little fear of having controlled or uncontrolled nuclear related goods stopped prior to reaching North Korea,” Mr. Albright said. “One hopes the Trump administration will succeed in pressing China to take the types of enforcement steps against commodity trafficking taken years ago in the West.”