President Donald Trump on Thursday threatened “some pretty severe things” against the North Korean regime for its nuclear test earlier this week, without offering any specifics while saying “something will have to be done about it.”

“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 during a joint press conference with Polish President Andrzej Duda in Warsaw. “They are behaving in a very, very dangerous manner.”

“We’ll just take a look at what happens over the coming weeks and months,” Trump added, without providing any details.

North Korea on Monday conducted a successful test of an intercontinental ballistic missile (ICBM), a weapon capable of reaching as far as Alaska, escalating already dire diplomatic tensions.

Over the last few days, Trump tweeted several tirades complaining about what he perceived as insufficient diplomacy toward North Korea, singling out China, its only ally, for not enacting strict economic punishments. The president also wondered: “Does this guy have anything better to do with his life?” referring to the country’s dictator Kim Jong Un, and suggested South Korea and Japan should not “put up with this much longer.”

Trump’s Twitter rants came as numerous U.S. diplomatic posts remain vacant or only temporarily filled, including the ambassador to South Korea, as well as the assistant secretaries that oversee East Asia policy and nuclear nonproliferation.

As Trump remained vague about how he plans to deal with North Korea, former U.S. Defense Secretary William Perry expressed doubts that any diplomatic solution is possible with the regime. But Perry, who headed the Pentagon from 1994 to 1997 for President Bill Clinton, also warned that the alternatives to diplomacy “are really very grim.”

During Thursday’s press conference, Trump also continued to cast doubt on Russian interference in last year’s U.S. election, saying “nobody really knows for sure” ― even though top U.S. intelligence agencies have affirmed that Russian President Vladimir Putin’s government sought to influence the outcome of the 2016 vote.

Trump is scheduled to meet with Putin on Friday during the G-20 summit in Hamburg, Germany. North Korea is expected to loom large during the meeting of leaders from the world’s 20 major economies.