As a U.S. Navy “armada” steamed toward the Korean Peninsula on Wednesday, Chinese leader Xi Jinping spoke with U.S. President Donald Trump over the phone, reiterating the need for a peaceful resolution to the North Korean nuclear standoff.

“China insists on realizing the denuclearization of the peninsula … and is willing to maintain communication and coordination with the American side over the issue on the peninsula,” Xi said, according to state broadcaster CCTV and other official media outlets.

The phone call, which China’s Foreign Ministry said had been initiated by Trump, came amid growing concerns that the U.S. was eyeing military action to rein in the North’s nuclear weapons and missile programs.

On Tuesday, Trump had dangled the offer of a “far better” trade deal with China if Beijing “solves” the nuclear issue, but said the United States was willing to go it alone amid surging tensions in Northeast Asia.

“I explained to the President of China that a trade deal with the U.S. will be far better for them if they solve the North Korean problem!” Trump wrote on Twitter in reference to his summit last week with Xi, the two leaders’ first face-to-face meeting.

Just minutes later, the U.S. president, who has touted his abilities as a deal-making businessman and had slammed Beijing as “raping” America on trade, took a tougher line.

“North Korea is looking for trouble. If China decides to help, that would be great. If not, we will solve the problem without them! U.S.A.,” Trump wrote.

Trump has repeatedly said China has the power to bring the North to heel over its nuclear weapons program, including in an interview with the Financial Times earlier this month.

“China has great influence over North Korea. And China will either decide to help us with North Korea, or they won’t. And if they do that will be very good for China, and if they don’t it won’t be good for anyone,” Trump said, according to an edited transcript published by the newspaper.

Those remarks came ahead of the summit with Xi, who vowed at the talks to bolster cooperation with the U.S. on the North issue. Beijing, however, has persistently called for multilateral talks to resolve the standoff and remains staunchly opposed to the use of force.

Tensions on the Korean Peninsula have soared in recent months as the North has continued its tests of powerful missiles and engines despite United Nations sanctions prohibiting them from doing so.

Asked about the tweets, White House spokesman Sean Spicer said Trump had put Pyongyang “on notice” that its missile tests were “not tolerable.”

“The last thing we want to see is a nuclear North Korea that threatens the coast of the United States, or, for that matter, any other country,” Spicer said. “So we need stability in that region, and I think he has put them clearly on notice.”

Experts called Trump’s Twitter diplomacy “dangerous” and often factually incorrect.

Trump’s “tweets are dangerous as they are often plain wrong in terms of contents,” said Stephen Nagy, a senior associate professor of politics at International Christian University in Tokyo. Nagy cited tweets by Trump claiming his predecessor, President Barack Obama, had wiretapped him, among other dubious claims.

“It plays well to his supporters but demonstrates the disconnect between policy and these 140 character pronouncements,” Nagy added. “They also contradict his national security team and long-held U.S. approaches to diplomacy.

“China already has shifted its approach to tweets, focusing on patience and observation of action rather than tweets,” he said.

Kerry Brown, a professor of China studies at King’s College in London, said any kind of deal with the U.S. on North Korea was a long shot.

“There is no deal that China can do on North Korea,” Brown said. “It regards it as in its strategic space. An attack on North Korea is therefore an attack on China.”

Brown called attention to the fact that while the current Chinese leadership may view Pyongyang with disdain, North Korea is China’s only treaty ally.

“It was willing to go to war with North Korea in 1950 when the People’s Republic of China had just been established,” he said. “That history is a deep and close one. … The history matters. They would be horrified at the idea of an America being able to unilaterally interfere in their most intimate strategic space. They would not allow this to happen.”

Trump’s Twitter barrage came a day after North Korea blasted the United States for rerouting the USS Carl Vinson-led aircraft carrier strike group to waters off the Korean Peninsula, saying it is “ready to react to any mode of war.”

The state-run Korean Central News Agency quoted a Foreign Ministry spokesman as saying that the dispatch of the strike group showed that “the U.S. reckless moves for invading the DPRK have reached a serious phase of its scenario.”

DPRK is the acronym for the North’s formal name, the Democratic People’s Republic of Korea.

“If the U.S. dares opt for a military action, crying out for ‘preemptive attack’ and ‘removal of the headquarters,’ the DPRK is ready to react to any mode of war desired by the U.S.,” the spokesman said.

The U.S. Navy’s Pacific Command re-routed the carrier group, which departed Singapore on Saturday, from planned port calls in Australia amid Pyongyang’s spate of recent missile launches and apparent signs of a sixth nuclear test. The Carl Vinson strike group, led by the Nimitz-class aircraft carrier of the same name, includes two guided-missile destroyers and a guided-missile cruiser.

Trump described the strike group in an interview Tuesday with the Fox Business Network as “an armada.”

“We are sending an armada. Very powerful,” he said. “We have submarines. Very powerful. Far more powerful than the aircraft carrier. That I can tell you.”

Carrier strike groups are generally accompanied by submarines, although this is normally not publicized by the Pentagon.

Referring to North Korean leader Kim Jong Un, Trump said: “He is doing the wrong thing.” Asked if he thought Kim was mentally fit, Trump replied: “I don’t know. I don’t know him.”

Separately, Spicer, the White House spokesman, called the strike group’s re-routing “prudent” — a word that has become the Trump administration’s go-to phrase in describing the move.

“It does a lot of things,” Spiced said. “It ensures … we have the strategic capabilities, and it gives the president options in the region.”

Defense Secretary James Mattis echoed Spicer on Tuesday, calling the decision the “most prudent” at this time. Mattis, however, said there was not “a specific … signal or specific reason why we’re sending her up there,” a message that clashed with earlier statements from the U.S. Navy’s Pacific Command, which had alluded to the North’s provocations as the reason behind the change in plans.

The decision to re-route the Vinson group came just days after U.S. cruise missiles struck Syria. Those strikes were widely interpreted as an implicit message to Pyongyang that the White House is not ruling out unilateral military attacks on the Kim regime.

The Trump administration has said that all options — including military action — are on the table.

While U.S. carrier deployments to the Western Pacific are not unusual, the Carl Vinson visited South Korea just last month for annual joint military exercises. South Korean media reports said the strike group was due to arrive the same week Pyongyang marks the April 15 anniversary of North Korean founder Kim Il Sung’s birth, an event where it is likely to flaunt its military might, possibly with its sixth nuclear test or a military parade showcasing an intercontinental ballistic missile (ICBM).

The North is also set to mark the army’s foundation day on April 25. Pyongyang has a long history of using such anniversaries to flex its military muscle.

According to analyses of recent commercial satellite imagery, the North has also been making apparent preparations for an atomic test.

China’s state-run Global Times newspaper, which is known for its strident nationalistic tone, said in an editorial that a new nuclear test would be a “slap in the face of the U.S. government,” adding that Beijing would not “remain indifferent” to further provocations.

Pyongyang has so far staged five nuclear tests — two of them last year, including its most powerful to date.

“Presumably Beijing will react strongly to Pyongyang’s new nuclear actions,” the newspaper said, adding there was increasing popular support for “severe restrictive measures that have never been seen before.”

The stark language suggested Beijing, Pyongyang’s only major ally and its economic lifeline, could put a chokehold on the North by restricting oil imports to the North.

“The U.S. is making up its mind to stop the North from conducting further nuclear tests, it doesn’t plan to co-exist with a nuclear-armed Pyongyang,” the paper said.

“Pyongyang should avoid making mistakes at this time,” it added.

There has been growing speculation that Pyongyang will conduct an ICBM test soon — possibly this month — after Kim used a New Year’s Day address to claim that the North was in the final stages of developing such a weapon.

Last week, the North test-fired a missile into the Sea of Japan that was believed to have traveled just “tens of kilometers,” the Japanese government said.

That launch was the latest in a spate of tests this year, including the near-simultaneous firing of four ballistic missiles into the Sea of Japan last month — a move the North said was a rehearsal for attacking U.S. bases in Japan. Those missiles, three of which fell into Japan’s exclusive economic zone, flew about 1,000 km. Abe characterized that test as “a new level of threat.”

Missile experts said the hypothetical target of that drill appeared to be U.S. Marine Corps Air Station Iwakuni in Yamaguchi Prefecture. Observers said the undisguised threat to U.S. bases in Japan was rare, even for Pyongyang, which routinely serves up colorful invectives.

Pyongyang has conducted more than 20 missile launches and two nuclear tests over the past year as it seeks to master the technology needed to mount a warhead on a long-range ballistic missile capable of striking the continental United States.