North Korea has been making bellicose threats against South Korea, Japan and US bases in the region
The US says it is urging China to use all its leverage to help rein in North Korea's "destabilising" actions.
US officials travelling with Secretary of State John Kerry said Washington wanted Beijing to evoke "a sense of urgency" in its talks with Pyongyang.
North Korea has ratcheted up tensions in the region, threatening nuclear strikes against South Korea and the US.
Meanwhile, the Pentagon has denied a report suggesting Pyongyang is able to mount nuclear warheads on missiles.
Earlier, a US Congressman leaked details of a Defense Intelligence Agency study that, he said, assessed "with moderate confidence" the North could fire a nuclear-armed missile, though with "low reliability".
Pentagon spokesman George Little said later: "It would be inaccurate to suggest that the North Korean regime has fully tested, developed or demonstrated the kinds of nuclear capabilities referenced in the passage."
Although the North has carried out nuclear tests, it had been thought it lacked the ability to make the weapons small enough to place in a ballistic missile.
South Korea is currently on a high state of alert amid indications that the North is preparing for a missile test.
Pyongyang has moved two Musudan ballistic missiles to its east coast. Estimates of their range vary, but some suggest the missiles could travel 4,000km (2,500 miles).
That would put US bases on Guam within range, although it is not known whether the Musudan has been tested before.
Mr Kerry is travelling to Seoul on his first trip to Asia since becoming secretary of state.
A senior administration official told reporters on board his plane: "It is no secret that China has most leverage, most influence, with North Korea and I think fundamentally we would want them to use some of that leverage because otherwise it is very destabilising and it threatens the whole region."
The official added that, although Washington was not privy to conversations between China and North Korea, "we would want China to bring a sense of urgency, the need to stop this escalation, into that debate".
"China has a huge stake in stability and the continued North Korean pursuit of a nuclear armed missile capability is the enemy of stability. That gives us and the Chinese a very powerful objective in common, namely denuclearisation," the official said.
US President Barack Obama has urged Pyongyang to end its "belligerent approach... and to try to lower temperatures".
But he warned that while he preferred to see tensions resolved through diplomatic means, "the United States will take all necessary steps to protect its people".
North Korea has increased its warlike rhetoric following fresh UN sanctions imposed after its third nuclear test in February and joint military manoeuvres by the US and South Korea.
The North says it will restart a mothballed nuclear reactor, has shut an emergency military hotline to the South and has urged countries to withdraw diplomatic staff, saying it cannot now guarantee their safety.
However, in the past few days North Korea's media appear to be in more of a holiday mood, due to the approach of Monday's celebrations marking the birth of national founder Kim Il-sung - a potential launch date for a new missile test.
On Thursday, foreign ministers from the G8 group of nations condemned in the "strongest possible terms" North Korea's nuclear weapons and ballistic missile programmes.
Following talks in London, UK Foreign Secretary William Hague said that if the North conducted another missile launch or nuclear test "we have committed ourselves to take further significant measures".