North Korea on Tuesday test-launched an intercontinental ballistic missile that some experts believe has the range to reach the U.S. states of Alaska and Hawaii and perhaps the U.S. Pacific Northwest. North Korea said it could carry a large nuclear warhead.
Speaking at a news conference with Polish President Andrzej Duda, Trump said Korea was "a threat, and we will confront it very strongly".
He said the United States was considering "severe things" for North Korea, but that he would not draw a "red line" of the kind that his Democratic predecessor, Barack Obama, had drawn but not enforced on the use of chemical weapons in Syria.
Trump added: "... they are behaving in a very, very dangerous manner and something will have to be done."
The issue presents Trump, who took office in January, with perhaps his biggest foreign policy challenge. It has put pressure on his relationship with Chinese President Xi Jinping, whom Trump had pressed without success to rein in Pyongyang.
The United States said on Wednesday that it was ready to use force if necessary to stop North Korea's nuclear missile program. But China on Thursday called for restraint and made clear it did not want to be targeted by U.S. sanctions.