He called on world leaders to show North Korea there are consequences "for their very, very bad behaviour"
Donald Trump has said he has "pretty severe things" in mind to confront North Korea if it does not end 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 a ballistic missile earlier 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.
Earlier this week, Mr Trump 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.
Trade between China and North Korea grew almost 40% in the first quarter. So much for China working with us - but we had to give it a try!— Donald J. Trump (@realDonaldTrump) July 5, 2017
US officials described it as intensifying the threat against the US because of the possibility that a nuclear warhead could be mounted on top of the missile and could reach American soil.
He 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 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."
The US President is due to give a keynote address from Krasinski Square - site of a monument that commemorates the 1944 Warsaw Uprising against the Nazis.
He will 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 is being hosted by Poland's first lady Agata Kornhauser-Duda for tea at the Belvedere palace during Mr Trump's 16-hour visit.
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.