The Trump administration believes China is using "back channel networking" with North Korea in a bid to persuade Kim Jong Un to stop nuclear and ballistic missile tests, the United Nations’ US ambassador has said.
"We believe they are being productive," Nikki Haley told reporters.
"We do think they’re trying to counter what is happening now."
Ms Haley said China knew North Korea best "and so we’re going to keep the pressure on China, but we’re going to continue to work with them in any way that they think is best".
At the same time, Ms Haley said, the United States and China are discussing the timing of a new UN Security Council resolution that would toughen sanctions against North Korea in response to its latest ballistic missile launches.
Monday’s launch was the third in three weeks.
Beijing is North Korea’s traditional ally and accounts for up to 90% of the isolated nation’s external trade, giving it considerable economic leverage.
But there are limits to China’s influence and its willingness to use it.
North Korea has advanced its nuclear weapons programme over Chinese objections.
But despite US calls for China to turn the screw, Beijing remains reluctant to impose biting economic pressure as it fears a North Korean collapse that would lead to instability on China’s doorstep.
China’s UN ambassador Liu Jieyi made clear last week that Beijing’s top priority is to restart talks with North Korea following its multiple tests to try to reduce tensions rather than impose new sanctions.
He stressed that all progress with Pyongyang on eliminating nuclear weapons from the Korean peninsula had come through dialogue.
Ms Haley said Washington and Beijing were trying to decide the best way to approach North Korea.
"I don’t think it’s back-pedalling as much as nothing is changing North Korea’s actions," she said.
"If this is going to happen every other day, how should we respond in a way that we actually stop these things, or slow it down?
"I think we’re having those conversations this week, and I hope that we can come up with a final solution."
Earlier this month Ms Haley said Beijing was not engaging on a new resolution, but told reporters on Tuesday that she had heard from the Chinese and discussions were now about "at what point do we do the resolution".
The Security Council has imposed six rounds of increasingly tough sanctions on North Korea.
Pyongyang’s reaction has been defiance and stepped up testing to improve its nuclear arsenal and achieve its goal of being able to launch an intercontinental ballistic missile with a nuclear warhead capable of reaching the United States.
In a sign of growing US anxiety over North Korea’s weapons development, the Pentagon said on Tuesday it had shot down a mock warhead over the Pacific.
It was the first test in three years of a US-based missile defence system and the first targeting an intercontinental-range missile like the one North Korea is developing.
North Korea’s deputy UN ambassador Kim In Ryong said on May 19 that his government would rapidly strengthen its nuclear strike capability as long as the United States maintained its "hostile policy" towards the country.
He said if the Trump administration wants peace on the Korean peninsula it should replace the armistice agreement that ended the 1950-53 Korean War with a peace accord and halt its anti-North Korea policy, which he called "the root cause of all problems".
The US has said there should be no talks until North Korea takes steps towards getting rid of its nuclear arsenal.
North Korea has shown great disrespect for their neighbor, China, by shooting off yet another ballistic missile...but China is trying hard!— Donald J. Trump (@realDonaldTrump) May 29, 2017