A Chinese statement issued during a visit by US Secretary of State John Kerry said the nuclear issue was the "shared responsibility of all parties".
Mr Kerry said the two sides must decide "very quickly" how to proceed.
North Korea has recently threatened nuclear attacks, and is feared to be preparing a missile launch.
A flurry of warlike statements from Pyongyang has prompted speculation that a launch could happen on 15 April, when the country marks the 101st birthday of the nation's founder and former leader, Kim Il-sung.
By Damian GrammaticasChina correspondent
After weeks of rising tensions, and North Korea's threats of nuclear war, the diplomatic efforts to tackle this crisis are gathering pace.
The pledge to work with the US is a clear signal of China's displeasure with North Korea. China is North Korea's only ally, and is not going to stop supporting the North, but wants it to calm things down.
The question is will North Korea listen? In recent weeks the North's  regime has made clear it now views having nuclear weapons as vital to its survival.
It has said explicitly that it won't bargain them away.
North Korea has reportedly moved at least two Musudan ballistic missiles to its east coast, but on Saturday, South Korea's Yonhap news agency quoted officials in Seoul as saying that no new movement of the mobile launchers had been detected for two days.
US officials including Mr Kerry have been playing down a leaked Pentagon report which warned that Pyongyang could have developed the technology to mount a nuclear warhead on a missile.
Since the UN imposed fresh sanctions on North Korea in February, its leadership has promised to restart a mothballed nuclear reactor, has shut an emergency military hotline to South Korea, and has urged diplomatic staff to leave, saying it cannot guarantee their safety.
The North says it has also been angered by joint US-South Korean military exercises.
Though its rhetoric has been more bellicose than usual, analysts say it fits a long-standing pattern, and may be intended to boost the popularity of Kim Jong-un, who came to power last year.
In Beijing Mr Kerry held talks with President Xi Jinping and Foreign Minister Wang Yi. In comments to Mr Xi, he said the world was facing "a critical time with some very challenging issues".
Among them were Korean tensions but also "the challenge of Iran and nuclear weapons, Syria and the Middle East, and economies around the world that are in need of a boost", he said.
He later issued a statement saying the US and China were able "to underscore our joint commitment to the denuclearisation of the Korean peninsula in a peaceful manner".
He said the two sides would have "further discussions to bear down very quickly with great specificity on exactly how we will accomplish this goal".
Missile defences in the region
North Korea's missile programme
The statement was issued after talks with Yang Jiechi, China's top diplomat.
Mr Yang said China was "firmly committed to upholding peace and stability and advancing the denuclearisation process".
"To properly address the Korea nuclear issue serves the common interests of all parties. It is also the shared responsibility of all parties," he said.
China is North Korea's only ally and major trading partner, but has grown increasingly frustrated with its growing belligerence.
Even so, the BBC's Damian Grammaticas reports from Beijing that persuading China to up the pressure on North Korea will not be easy.
China might be frustrated by Pyongyang's behaviour but it is still the North's lifeline, so Beijing is unlikely to cut vital supplies of food and fuel since it feels that may make the North even more desperate and unpredictable, our correspondent adds.
Following meetings in the South Korean capital, Seoul, on Friday, Mr Kerry said no country had a closer relationship with Pyongyang than China.
Noting that Beijing, like Washington, wanted denuclearisation on the peninsula, he added: "If that's your policy, you've got to put some teeth into it."
He warned North Korea against any missile launch, saying it would be a "provocation and unwanted act" which would further isolate North Korea and its people who, he said "are desperate for food, not missile launches".
But he also said the US had tried to tone down its own statements, and cancelled some military exercises.
"I think we have lowered our rhetoric significantly and we are attempting to find a way for reasonableness to prevail here," he said.
Mr Kerry is on a four-day trip to Asia, and travels to Tokyo on Sunday.
Russia has also expressed growing concern over North Korea and said on Friday that it had issued "an urgent appeal" to Pyongyang  "to refrain from actions which could lead to further escalation of tension".
Some estimates suggest that the missiles North Korea has moved to its east coast could travel 4,000km (2,500 miles), although it is not believed that the Musudan has been tested before.
That would put US bases on the Pacific island of Guam within range.
China and the US have vowed to work together to persuade North Korea to give up its nuclear programme and to settle tensions through dialogue.