Donald Trump has said he has "pretty severe things" in mind to confront North Korea if it does not end its nuclear weapon ambitions.
Speaking during a joint news conference with Poland President Andrzej Duda in Warsaw, Mr Trump called on other nations to address "very, very bad behaviour" from Pyongyang after it tested an intercontinental ballistic missile (ICBM) this week.
The US President said North Korea had been behaving in a "very, very dangerous manner", adding: "I call on all nations to confront this global threat and publicly demonstrate to North Korea that there are consequences for their very, very bad behaviour.
"I have pretty severe things that we're thinking about. That doesn't mean that we'll do them."
Washington has been contemplating a series of possible sanctions and economic measures against Pyongyang in response to the firing of an ICBM .
:: US 'prepared to use force' against N Korea
Mr Trump on Wednesday expressed his frustration with China for continuing trade with North Korea and failing to apply pressure on Kim Jong Un to halt the regime's nuclear weapons programme.
He wrote: "So much for China working with us - but we had to give it a try!"
The US has said it is prepared to use force against North Korea amid fears Pyongyang may put a nuclear warhead on top of a missile that could reach American soil.
Mr Trump has also criticised Russian actions and "destabilising behaviour" in the region as he praised Poland as "not only a great friend but a truly important ally".
Around 900 US troops are stationed in Poland as part of a rotating NATO force in eastern Europe to counter potential threats from Russia.
Mr Trump praised his counterpart for exceeding Poland's required contribution to NATO.
Mr Duda said: "We see ourselves as loyal partners who co-operate on a number of issues, among others on security.
"I have a feeling that the United States is serious about Poland's security."
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The US President gave a keynote address from Krasinski Square - site of a monument that commemorates the 1944 Warsaw Uprising against the Nazis - as he was cheered by the crowd.
He said: "Just as Poland could not be broken, I declare today for the world to hear that the West will never, ever be broken.
"Our values will prevail, our people will thrive and our civilisation will triumph."
Mr Trump also meet the heads of a dozen countries bordering the Baltic, Adriatic and Black seas. The group is collectively known as the Three Seas Initiative and aims to reduce the region's dependence on Russian energy.
Melania Trump also addressed the crowd in Warsaw, after she was hosted by Poland's first lady Agata Kornhauser-Duda for tea at the Belvedere palace.
Following his visit to Poland, Mr Trump will attend the G20 summit in Germany, where he will hold face-to-face talks with Chancellor Angela Merkel and hold his first meeting with Russian President Vladimir Putin.
He has been reluctant to adopt a harder line towards Russia and has seen his administration embroiled in continuing federal investigations into possible collusion with Russian government officials.
On Thursday, Foreign Secretary Boris Johnson said Mr Trump's approach to politics, including his controversial use of Twitter, has "gripped the imagination" of the world.
But he insisted the UK was helping to "mitigate" the White House approach.
He told the BBC: "Our role, as the UK is to represent our own point of view, whether it is on NATO, the vital importance of Article 5 of NATO, whether it's on climate change, whether it's on the Iran nuclear accord - it's the UK that is actually helping, we think, to mitigate, to get some of those American attitudes and polices that are currently coming out of the White House into a better place."
The UK summoned the North Korea ambassador on Wednesday to condemn the missile launch.