The US Secretary of State, Rex Tillerson, has called on the rest of the world to help force North Korea to give up its nuclear ambitions.
Speaking to the UN Security Council, he called on China in particular to leverage its trade links as influence.
But the Chinese Foreign Minister, Wang Yi, said the key to solving the problem did not lie with his country.
Tensions have increased lately, with both North and South Korea conducting military exercises.
The US has sent warships and an anti-missile system to reassure the South, where thousands of American soldiers are stationed.
North Korea has made repeated attempts to miniaturise nuclear warheads and fit them on long-range missiles capable of reaching the US.
Mr Tillerson warned the UN Security Council in New York of "catastrophic consequences" if it did not act, saying it was "likely only a matter of time before North Korea develops the capability to strike the US mainland". The US would use military force if necessary, he added.
He urged other countries to suspend diplomatic ties with Pyongyang and isolate its financial institutions.
In comments that seemed directed at China, he accused Council members of not fully enforcing existing sanctions against the North.
What are the sanctions on North Korea?
UN sanctions include a ban on selling North Korea arms, fuel, a host of items that could be used for weapons-making.
Also on the list are luxury goods including pearl jewellery and snowmobiles worth $2,000 (£1,540) or more.
Since last year, all cargo entering or leaving North Korea must also be inspected.
But a recent UN study found that fragments from a North Korean missile test included electronics that had been sourced either from or via Chinese enterprises.
The US has separate, stricter sanctions including a blanket ban on trade and a blacklist of anyone dealing with North Korea.
What did the US and China say?
Asked on Friday by US broadcaster NPR if the US was prepared to hold direct talks with North Korea, Mr Tillerson replied: "Obviously, that would be the way we would like to solve this. But North Korea has to decide they're ready to talk to us about the right agenda."
China's foreign minister warned against military intervention, saying: "The use of force does not solve differences and will only lead to bigger disasters."
"Peaceful settlement of the nuclear issue on the Korean peninsula through dialogue and negotiations represents the only right choice that is practical and viable," Mr Wang added.
He also repeated a Chinese proposal for a halt to Pyongyang's military programme in return for a freeze on joint US-South Korea military drills.
The US has rejected the idea in the past, saying the nuclear programme must stop first.
Analysis by Barbara Plett-Usher in Washington
What is this new approach the secretary of state wants to see from the United Nations?
He laid out three elements: strictly enforce existing sanctions, impose new ones, and isolate North Korea diplomatically.
And he signalled greater US resolve to pursue this agenda. Most significantly he threatened US sanctions against entities and individuals in other countries that support North Korea's illegal activities. That may be putting Chinese banks on notice.
He also called UN members to downgrade or suspend diplomatic relations - hoping to cut North Korea's useful links with nations like Cambodia, Laos and Malaysia.
And he clarified the stakes: the new US campaign is driven by its own national security considerations, he said.
So it's serious.
The level of concern on the Security Council will be tested if North Korea conducts another nuclear or big missile test; that would be the most likely trigger for any move to new sanctions.
What have other countries said?
- Russian Deputy Foreign Minister Gennady Gatilov said the use of force would be "completely unacceptable" but he also called on North Korea to end its nuclear and missile programmes
- UK Foreign Secretary Boris Johnson told the Security Council that while negotiations "must at some point form part of the solution", North Korea should first "make verifiable progress" towards denuclearisation.
How else has tension risen?
Among other developments in recent weeks:
- North Korea carried out a failed missile launch and held a massive military parade in an apparent show of strength
- The US deployed a group of warships and a submarine to the region
- Pyongyang reacted angrily, threatening a "super-mighty pre-emptive strike"
- The US began installing a controversial $1bn (£775m) anti-missile system system called Thaad in South Korea - which Mr Trump said South Korea should pay for. Seoul said on Friday there was "no change" in its position that the US pays for it
- Mr Tillerson and US Vice President Mike Pence visited South Korea, reiterating that "all options were on the table" in dealing with the North