President Donald Trump labeled North Korea's behavior a threat and promised, "we will confront it very strongly."
North Korea carried out a test launch Tuesday of its first intercontinental ballistic missile, complete with a re-entry vehicle that would allow it to be equipped with a nuclear warhead.
U.S. military officials estimated the missile had a range of 5,500 kilometers, potentially putting parts of the northwestern United States within Pyongyang's reach.
Speaking at a news conference Thursday in Warsaw with Polish President Andrzej Duda, Trump warned North Korea he is considering "some pretty severe things."
"They are behaving in a very, very dangerous manner and something will have to be done," he said. "There are consequences for their very, very bad behavior."
The U.S. president did not elaborate on which options are under consideration, but Defense Secretary Jim Mattis told reporters later Thursday at the Pentagon there was still time for a diplomatic solution despite North Korea's continued provocations.
"I do not believe this capability in itself brings us closer to war because the president's been very clear, the secretary of state's been very clear, that we are leading with diplomatic and economic efforts," he said.
"We are working with allies. We are working with the Chinese," Mattis added. "But obviously any kind of effort by North Korea to start a war would lead to severe consequences."
WATCH: 'Severe Things' Considered for North Korea, Trump Says
Mattis said the U.S. military has readied several options in case diplomacy ultimately fails but noted U.S. defenses were not being buttressed as a result of the ICBM test and that the launch itself did not take the Pentagon by surprise.
"We knew it as soon as he [North Korea's Kim Jong Un] fired it," said Mattis. "We assume these sorts of things from him right now."
The United States has 28,500 troops stationed in South Korea, and about 50,000 in Japan.
The U.S. and South Korea also have deployed some components of the Terminal High Altitude Area Defense, or THAAD, anti-missile system in South Korea. Other missile defenses include the Aegis ballistic missile defense system, which can be deployed on naval vessels.
And, in a test last month, a U.S.-based missile interceptor knocked down a simulated incoming North Korean ICBM. Other tests of the system, however, have met with mixed results.
"We do have confidence in our ability to defend against the limited threat, the nascent threat that is there," Pentagon spokesman Capt. Jeff Davis said Wednesday.
On Wednesday, U.S. Ambassador to the United Nations Nikki Haley said the U.S. is prepared to defend itself and its allies.
"One of our capabilities lies with our considerable military forces," she told an emergency meeting of the Security Council. "We will use them if we must, but we prefer not to have to go in that direction."
Haley said the United States will propose new sanctions against North Korea shortly. A new draft resolution will be circulated among Security Council members "in the coming days," she noted, adding the measure "raises the international response in a way that's proportionate to North Korea's escalation."
Among new sanctions under consideration, U.S. officials indicated, are restrictions on the flow of oil and other energy supplies to Pyongyang's military and weapons programs, tightened controls over air and maritime traffic to North Korea, and further moves to hold senior officials of the Kim Jong Un regime accountable for the country's defiance of international demands to shut down its nuclear-weapons development program.
The U.S. and China remain far apart on what to do about North Korea, and Beijing's response to the next U.S. move on North Korea will be key to the success of the American effort.
North Korean leader Kim Jong Un boasted Wednesday about the missile launch.
He told a group of scientists and technicians the new rocket was "a package of gifts" timed to coincide with the U.S. Independence Day holiday.
The regime's Korean Central News Agency said Kim expected the U.S. would be "displeased" by Pyongyang's achievement, and he urged his scientists to conduct further tests as "big and small 'gift packages' to the Yankees."
Margaret Besheer, Joyce Huang, Saibal Dasgupta and Victor Beattie contributed to this report.
