US Secretary of State Rex Tillerson has said that China has threatened to impose sanctions on North Korea if it conducts further nuclear tests.
"We know that China is in communications with the regime in Pyongyang," Mr Tillerson said on Fox News Channel. "They confirmed to us that they had requested the regime conduct no further nuclear test."
Mr Tillerson said China also told the US that it had informed North Korea "that if they did conduct further nuclear tests, China would be taking sanctions actions on their own".
Earlier on Thursday, the senior US Navy officer overseeing military operations in the Pacific said the crisis with North Korea is at the worst point he has ever seen, but he declined to compare the situation to the Cuban missile crisis decades ago.
"It's real," Admiral Harry Harris Jr commander of US Pacific Command, said during testimony before the Senate Armed Services Committee.
Admiral Harris said he has no doubt that North Korean leader Kim Jong Un intends to fulfil his pursuit of a nuclear-tipped missile capable of striking the United States. The admiral acknowledged there is uncertainty within US intelligence agencies over how far along North Korea's nuclear and missile programmes are. But Harris said it is not a matter of if but when.
"There is no doubt in my mind," Admiral Harris said.
China's foreign ministry had no immediate comment on Mr Tillerson's remarks on new sanctions, but a spokesman said Beijing remained committed to implementing sanctions imposed under UN Security Council resolutions.
"And the reason that China implements relevant resolutions is to fulfil our due international obligation rather than being pressured by any outside parties," Geng Shuang said at a daily briefing.
China has consistently called for an end to North Korea's nuclear weapons programme, but says it opposes unilateral sanctions imposed without a UN mandate. China in January suspended coal imports from the North for the rest of the year, but it did so following the passage of a Security Council resolution capping Pyongyang's coal exports.
Other economic activity with North Korea remains robust.
Still, Beijing has been increasing pressure on North Korea, and would be willing to impose punitive measures unilaterally in the event of another nuclear test, said Cheng Xiaohe, an associate professor at Renmin University's School of International Studies in Beijing.
"So Tillerson's comments are correct - but be careful, China does so for China's national interest, not as a result of US pressure," Mr Cheng said.
The Trump administration has declared that all options, including a targeted military strike, are on the table to block North Korea from carrying out threats against the United States and its allies in the region. But a pre-emptive attack is not likely, US officials have said, and the administration is pursuing a strategy of putting pressure on Pyongyang with assistance from China, North Korea's main trading partner and the country's economic lifeline.
With international support, the Trump administration said on Thursday it wants to exert a "burst" of economic and diplomatic pressure on North Korea that yields results within months to push the communist government to change course from developing nuclear weapons.
Susan Thornton, the acting top US diplomat for East Asia, said there is debate about whether Pyongyang is willing to give up its weapons programs. She said the US wants "to test that hypothesis to the maximum extent we can" for a peaceful resolution.
But signalling that military action remains possible, Ms Thornton told an event hosted by the Foundation for Defence of Democracies - the Washington think tank has advocated tougher US policies on Iran and North Korea - that the administration treats North Korea as its primary security challenge and is serious that "all options are on the table".
"We are not seeking regime change and our preference is to resolve this problem peacefully," Ms Thornton said, "but we are not leaving anything off the table."
Mr Tillerson took a similar stand in the Fox News interview, saying: "We do not seek regime change in North Korea... What we are seeking is the same thing China has said they seek - a full denuclearisation of the Korean peninsula."
In a separate interview with National Public Radio, Mr Tillerson said the US remains open to holding direct negotiations with North Korea.
"But North Korea has to decide they are ready to talk to us about the about the right agenda, and the right agenda is not simply stopping where they are for a few more months or a few more years and then resuming things," he said, according to excerpts of an interview that will air on Friday morning. "That's been the agenda for the last 20 years."