China’s president, Xi Jinping, has told Donald Trump in a phone call that all sides should avoid rhetoric or action that would worsen tensions on the Korean peninsula, according to Chinese state media.

Reports quoted Xi as saying: “At present, the relevant parties must maintain restraint and avoid words and deeds that would exacerbate the tension on the Korean peninsula.”

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.

Trump on Friday went further to turn the crisis into a personal battle of wills between him and Kim Jong-un, warning the North Korean leader he would “truly regret” hostile acts against US territory or US allies.

The warning came a few hours after an early morning tweet from the president that claimed US military options were “locked and loaded” for use if Pyongyang “acted unwisely”.

The tweet triggered worldwide alarm and a rebuke from the German chancellor, Angela Merkel, who said: “I consider an escalation of rhetoric the wrong answer.”

But Trump stood by his words when asked about them at his golf resort in New Jersey.

“I hope they are going to fully understand the gravity of what I said, and what I said is what I mean,” Trump said. “Those words are very, very easy to understand.”

He then issued an ultimatum to Kim Jong-un himself. “This man will not get away with what he’s doing,” he said. “If he utters one threat in the form of an overt threat ..or if he does anything in respect to Guam or anyplace else that’s an American territory or an American ally, he will truly regret it and he will regret it fast.”

The North Korean leadership has warned it will launch four missiles at the waters around US Pacific territory of Guam as a warning to the US if it persists with its practice sorties by long-range bombers based on the island.

Despite gung-ho language from the US president, there was no change in US deployments in the region or a change in the alert status of US forces. And it was reported on Friday that the Trump administration had reopened a channel of communication between US and North Korean diplomats at the UN.

According to the Associated Press, the “New York channel” had been broken off by North Korea in protest against sanctions in 2016, but it was revived this year between Joseph Yun, the US envoy for North Korea policy, and Pak Song-il, a senior North Korean diplomat at the country’s UN mission.

The US state department said it had no comment on the report. It had previously been reported that there had been diplomatic contacts about US detainees in North Korea, but the new AP account said the talks addressed wider issues, although such contacts had so far failed to moderate the exchange of threats between the leaders of the two countries.

Asked about the report on Friday, Trump said: “Well, we don’t want to talk about progress, we don’t want to talk about backchannels.

“We want to talk about a country that has misbehaved for many many years – decades, actually – through numerous administrations, and they didn’t want to take on the issue and I had no choice but to take it on, and I’m taking it on. And we’ll either be very, very successful quickly or we’re going to be very, very successful in a different way, quickly.”

As for Merkel’s criticism earlier in the day, Trump said: “Maybe she’s speaking for Germany; let her speak for Germany. She’s a friend of mine, she’s a very good person, a very good woman, she’s a friend of Ivanka. Perhaps she is referring to Germany. She’s certainly not referring to the United States, that I can tell you.”

In his comments, Trump made it clear that domestic political considerations played a role in shaping his rhetoric.

“If somebody else uttered the exact same words that I uttered, they’d say: ‘What a great statement, what a wonderful statement,’” Trump said. “But I will tell you, we have tens of millions of people in this country that are so happy with what I am saying, because they say, ‘Finally, we have a president that’s sticking up for our nation and frankly, sticking up for our friends and our allies.’”

Speaking to reporters in California, the US defence secretary, James Mattis, said a conflict on the Korean peninsula would be “catastrophic” and stressed that US diplomats should take the lead in resolving the crisis.

Mattis pointed to a UN security council vote at the weekend for more sanctions against North Korea as a sign that diplomacy was making progress.

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