President Trump warned of “some pretty severe things” in response to North Korea’s test this week of a long-range missile, raising the rhetoric over Pyongyang’s launch of what it called an intercontinental ballistic missile.    

“We’ll see what happens. I don’t like to talk about what we have planned, but I have some pretty severe things that we’re thinking about,” Trump said in Warsaw at a news conference with Andrzej Duda, the Polish president. “They are behaving in a very, very serious manner, and something will have to be done about it.”

Trump’s remarks come a day after Nikki Haley, his ambassador to the UN, warned that while the U.S. preferred a diplomatic approach to resolving the crisis over North Korea, the U.S. “is prepared to use the full range of our capabilities to defend ourselves and our allies.”

“One of our capabilities lies with our considerable military forces,” she said. “We will use them, if we must, but we prefer not to have to go in that direction.”

At issue is the launch this week by the Kim Jong Un regime of an ICBM that it said made it “a full-fledged nuclear power … capable of hitting any part of the world.” The Trump administration, in its public statements, appeared to confirm the missile tested Tuesday was, in fact, an ICBM, though other experts have been more skeptical. Still, it is believed the missile launched Tuesday—North Korea’s most-successful test ever—can strike Alaska. Pyongyang’s goal is to fit the ICBM with a nuclear warhead, a capability that would dramatically raise the threat it poses and provide the North with a deterrent against a possible U.S.-led attack.

Even without that weapon, however, the North has a range of artillery, medium-range missiles, and plutonium warheads with which it can target South Korea, Japan, and U.S. military interests in the region. Most experts agree that Kim’s regime could have a nuclear device that capable of striking the U.S. West Coast by the end of Trump’s first term. The president has said that “will not happen”—though it’s unclear, given the severe potential consequences of trying to prevent it, what options the U.S. has.

Trump, speaking in Warsaw, said the U.S. would confront the “threat from North Korea … very strongly,” adding the world needed to “demonstrate that there are consequences for their very, very bad behavior.”

But any diplomatic solution to the crisis goes through Beijing, and China, which is North Korea’s main backer, will almost certainly veto any strong diplomatic response to the missile’s launch. Haley, in her remarks at the UN on Wednesday, urged China to use its economic influence over Pyongyang to bring about a change in Kim’s behavior, but China, as well as Russia, say only diplomatic concessions by both sides can bring about change. In a joint statement, the leaders of the countries urged North Korea to stop its missile and nuclear tests, but also called on the U.S. and South Korea to rethink the deployment of U.S. weapons in the South—which Pyongyang regards as a provocation.