UPDATE 12.50: Boris Johnson has said the regime of Kim Jong Un is responsible for the crisis over North Korea's nuclear programme and must now "fix it".
In a series of posting on his Twitter feed, the Foreign Secretary said Britain was working with the United States and allies in the region to find a diplomatic solution to the stand-off between Pyongyang and Washington.
"The North Korean regime is the cause of this problem and they must fix it ," he said.
"The international community is shoulder to shoulder in ensuring North Korea stops its aggressive acts.
"We are working with the US and our partners in the region to bring this crisis to a diplomatic end."
Earlier: Chinese President Xi Jinping has made a plea for cool-headedness over escalating tensions between the US and North Korea.
In a phone conversation with US President Donald Trump on Saturday, he urged both sides to avoid words or actions that could worsen the situation.
The call came after Mr Trump unleashed a slew of fresh threats against North Korea, declaring the US military "locked and loaded" and warning North Korean leader Kim Jong Un that he "will regret it fast" if he takes any action against US territories or allies.
Mr Trump has pushed China to pressure North Korea to halt a nuclear weapons program that is nearing the capability of targeting the United States.
China is the North's biggest economic partner and source of aid, but says it alone cannot compel Pyongyang to end its nuclear and missile programs.
State-run China Central Television quoted Mr Xi as telling Mr Trump the "relevant parties must maintain restraint and avoid words and deeds that would exacerbate the tension on the Korean Peninsula".
But restraint was not the word of the day on Friday as Mr Trump sent out a cascade of unscripted statements, including what appeared to be another red line - the mere utterance of threats - that would trigger a US attack against North Korea and "big, big trouble" for Kim.
North Korea's Minju Joson newspaper, meanwhile, hit back at the US in an editorial on Saturday.
"The powerful revolutionary Paektusan army of the DPRK, capable of fighting any war the US wants, is now on the standby to launch fire into its mainland, waiting for an order of final attack," it said.
DPRK stands for North Korea's official name, the Democratic People's Republic of Korea.
The tough talk capped a week in which long-standing tensions between the countries risked abruptly boiling over.
New United Nations sanctions condemning the North's rapidly developing nuclear program drew fresh ire and threats from Pyongyang.
Mr Trump, responding to a report that US intelligence indicates Pyongyang can now put a nuclear warhead on its long-range missiles, vowed to rain down "fire and fury" if challenged.
The North then came out with a threat to lob four intermediate-range "Hwasong-12" missiles near Guam, a tiny US territory some 3,200 km (2,000 miles) from Pyongyang.
At the epicentre of the rhetoric, Mr Trump's New Jersey golf course, the president seemed to put Kim on notice, saying: "If he utters one threat in the form of an overt threat - which by the way he has been uttering for years and his family has been uttering for years - or he does anything with respect to Guam or any place else that's an American territory or an American ally, he will truly regret it and he will regret it fast."
Asked if the US was going to war, he said: "I think you know the answer to that."
But his comments did not appear to be backed by significant military mobilisation on either side of the Pacific, and an important, quiet diplomatic channel remained open. As a precaution, Japan deployed missile defence batteries under the path a North Korean missile might take.
Life on the streets of the North Korean capital, Pyongyang, also remained calm.
There have been no air raid drills or cars in camouflage netting as has been the case during previous crises. State-run media ensures that the population gets the North Korean side of the story, but doesn't convey any sense of international concern about the situation.
US officials say they will be going ahead with long-scheduled military exercises with South Korea. Pyongyang says it will be ready to send its missile launch plan to Kim for approval just before or as the drills begin.
Called Ulchi-Freedom Guardian, the exercises are expected to run August 21-31 and involve tens of thousands of American and South Korean troops on the ground and in the sea and air.
North Korea claims the exercises are a rehearsal for war, but Washington and Seoul say they are necessary to deter North Korean aggression.
Mr Trump began his Friday barrage with an especially fiery tweet: "Military solutions are now fully in place, locked and loaded, should North Korea act unwisely. Hopefully Kim Jong Un will find another path!"
He later retweeted a posting from US Pacific Command that showed B-1B Lancer bomber planes on Guam that "stand ready to fulfill USFK's #FightTonight mission if called upon to do so".
''Fight tonight" has long been the motto of US forces in South Korea to show they're always ready for combat on the Korean Peninsula.
Mr Trump also brushed away calls for caution from other world leaders, including Germany's Angela Merkel.
"I don't see a military solution and I don't think it's called for," Mrs Merkel said, calling on the UN Security Council to continue to address the crisis.
"I think escalating the rhetoric is the wrong answer," she added.
"Let her speak for Germany," Mr Trump responded when asked about the comment. "Perhaps she is referring to Germany. She's certainly not referring to the United States, that I can tell you."
By evening, he seemed to have mellowed a bit.
"Hopefully it'll all work out," Mr Trump said. "Nobody loves a peaceful solution better than President Trump."
Speaking to Guam Governor Eddie Calvo, he promised: "You are safe. We are with you a thousand percent."
ecretary of state and UN ambassador, the president suggested diplomacy could yet prevail.
"Hopefully it'll all work out," he said. "Nobody loves a peaceful solution better than President Trump."
Mr Trump announced he planned to hold another press conference in Washington on Monday.