U.S. President Donald Trump, well-known for his off-the-cuff remarks, hinted Tuesday at the possibility of military action against North Korea if necessary, despite emphasizing his “good relationship” with leader Kim Jong Un.

“We have the most powerful military we have ever had and we are by far the most powerful country in the world, and hopefully we do not have to use it. But if we do, we will use it. If we have to, we will do it,” Trump said during a meeting in London ahead of a summit with NATO members.

The U.S. leader had been asked about North Korea’s continued development of its nuclear weapons program — which he denied was taking place — and its repeated missile tests.

Instead, the mercurial Trump focused squarely on his historic meetings with Kim.

“I have a very good personal relationship” with Kim, he said, adding, “I am possibly the only one he has that kind of relationship with in the world.”

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Still, Trump also managed to bring back one of his monikers for the North Korean leader, who has in recent months conducted a spate of weapons tests, including short-range missiles that experts say are designed to evade South Korean and Japanese missile defenses.

“He definitely likes sending rockets up, doesn’t he? That’s why I call him ‘rocket man,'” Trump said.

The president had steered clear of that nickname since the two sat down for their historic first summit in Singapore in June last year.

Trump had been relatively mum on the North Korean nuclear issue in recent months, preoccupied with the threat of impeachment and his 2020 re-election campaign. But with a looming end-of-the-year deadline set by Kim for the U.S. to bring a “new way of calculation” to the negotiating table, the issue is likely to be on Trump’s mind in the coming weeks.

The nuclear negotiations between the U.S. and the North have effectively been deadlocked since working-level talks in early October ended with Pyongyang’s top negotiator, Kim Myong Gil, saying they had broken off “entirely due to the United States’ failure to abandon its outdated viewpoint and attitude.”

Since then, senior North Korean officials have repeatedly criticized the U.S. over its position in the talks.

One thing is for sure, Pyongyang is doing all it can to remind Trump of the deadline, including a statement by another top official earlier Tuesday warning that it is up to Washington to decide “what Christmas gift it will select” ahead of the deadline.

In the event the deadline passes without any kind of progress, some experts have said Kim could end his country’s self-imposed moratorium on nuclear tests and launches of intercontinental ballistic missiles capable of hitting U.S. cities.

Another option for getting Trump’s attention could be for the North to again lob an intermediate-range missile over Japan. Indeed, a top North Korean Foreign Ministry official warned Saturday that Tokyo could again see “a real ballistic missile” overflying the country “in the not distant future.”

In 2017, the North launched two separate intermediate-range missiles over Japan as Pyongyang underwent a massive expansion in its capabilities.

Trump also said his administration was pushing ahead with negotiations with allies South Korea and Japan to shoulder more of the cost of stationing U.S. troops in those countries.

He said Japan, “a wealthy nation,” and Prime Minister Shinzo Abe, whom he characterized as “a friend,” are “gonna do a lot,” in terms of cost-sharing.

He also said Seoul had last year agreed to pay nearly $500 million a year more for U.S. “protection,” and added the United States was now seeking additional cash commitments.

Asked if it was in the U.S. national security interest to have American forces stationed on the Korean Peninsula, Trump was noncommittal.

“It can be debated,” he said. “I can go either way. I can make arguments both ways.”

“But I do think this, I think if we’re going to do it, they should burden-share more fairly,” Trump said.