Tillerson's remarks came the day after Trump appeared to suggest China's Xi had come up short in efforts to lean on Kim Jong-Un's regime.
Washington: US Secretary of State Rex Tillerson on Wednesday urged Chinese officials to apply greater diplomatic and economic pressure on North Korea to force Pyongyang to rein in its nuclear weapons program.
Tillerson's remarks came after he and Pentagon chief Jim Mattis met with the Chinese visitors at the State Department, where the former general said he saw scope for a better defense relationship.
The extent to which Beijing can influence Pyongyang is key in trying to defuse the North Korea crisis, and Tillerson's remarks came the day after President Donald Trump appeared to suggest China's President Xi Jinping had come up short in efforts to lean on Kim Jong-Un's regime.
Calling North Korea the "top security threat" to the United States, Tillerson said China has a "diplomatic responsibility to exert much greater economic and diplomatic pressure on the regime if they want to prevent further escalation in the region."
Trump -- who frequently denounced China on the campaign trail -- has turned to Beijing to help pressure its ally North Korea, prompting concern among Asian partners that America might go easy on the South China Sea territorial dispute.
But on Tuesday, Trump sent a tweet suggesting Xi's efforts had not borne fruit -- a message he reiterated before supporters in Iowa.
"I do like President Xi," he told the crowd Wednesday evening. "I wish we would have a little more help with respect to North Korea, from China. That doesn't seem to be working."
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.
Tillerson said the US and Chinese officials had agreed that companies from both countries should not do business with any UN-designated North Korean entities.
- 'Unremitting efforts' -
Trump has made halting the North Korean nuclear threat his number one foreign policy priority.
On Wednesday the US 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.
In a letter notifying Congress of the move, Trump wrote that the "existence and risk of proliferation of weapons-usable fissile material on the Korean Peninsula" together with the "provocative, destabilizing, and repressive actions" of the Pyongyang regime "continue to constitute an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States."
In Beijing, officials insisted they have not given up hope of influencing Pyongyang.
"To resolve the Korean peninsula nuclear issue, China has been making unremitting efforts and we have been playing an important and constructive role," said foreign ministry spokesman Geng Shuang, while stressing China was not the "focus and the crux" of the crisis.
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 US ambassador.
But tensions remain -- particularly over China's building of artificial islands in disputed South China Sea waters. "Secretary Mattis and I were clear that the US position remains unchanged," Tillerson said.
Still, Mattis spoke in favor of greater communication with the Chinese military, including an officer exchange program to "improve transparency and mutual understanding."
- Warmbier death -
Despite international condemnation and sanctions, North Korea has a small nuclear arsenal and is developing nuclear-capable ballistic missiles that threaten Japan and South Korea -- and one day could even hit some US cities.
Washington has 28,000 troops deployed in South Korea and a naval armada in the region.
China has tightened controls on trade in North Korean coal, but many doubt it will fully enforce any sanctions that might threaten the stability of its unpredictable neighbor.
Separately, Mattis blasted Pyongyang's treatment of Otto Warmbier, a 22-year-old student who died on Monday after being evacuated in a coma from detention 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, (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.
"What you're seeing, I think, is the American people's frustration with a regime that provokes and provokes and provokes, and basically plays outside the rules."