North Korea abruptly ended a 10-week pause in its weapons testing Tuesday by launching what the Pentagon believes was an intercontinental ballistic missile, a move that will escalate already high tensions with Washington.
President Donald Trump, who was briefed while the missile was still in the air, later reacted to the launch by saying: "We will take care of it."
The president, meeting Republican politicians in Washington, said: "It is a situation that we will handle."
Pentagon spokesman Colonel Rob Manning said that the missile was launched from Sain Ni, North Korea, and travelled about 620 miles before landing in the Sea of Japan.
Japan said it may have landed within 200 nautical miles of its coast.
The launch is North Korea's first since it fired an intermediate range missile over Japan on September 15, and it appeared to shatter chances that the hiatus could lead to renewed diplomacy over the reclusive country's nuclear programme.
US officials have sporadically floated the idea of direct talks with North Korea if it maintained restraint.
An intercontinental ballistic missile test will be considered particularly provocative as it would signal further progress by Pyongyang in developing a weapon of mass destruction that could strike the US mainland, which President Donald Trump has vowed to prevent.
Seoul's Joint Chiefs of Staff said the US and South Korean militaries were analysing the launch data from the missile, which it said was fired from an area in a city close to North Korea's capital.
In response, it said South Korea conducted a "precision-strike" drill, without elaborating.
South Korea's presidential office said it was holding a National Security Council meeting at 6am on Wednesday local time to discuss the launch.
A week ago, the Trump administration declared North Korea a state sponsor of terrorism, further straining ties between governments that are still technically at war.
Washington also imposed new sanctions on North Korean shipping firms and Chinese trading companies dealing with the North.
North Korea called the terror designation a "serious provocation" that justifies its development of nuclear weapons.
Echoing the initial US assessment, Japan's defence minister Itsunori Onodera said the missile was likely an intercontinental ballistic missile.
"We can assume it was ICBM-class," Mr Onodera said.
The South Korea said the missile travelled a distance of 600 miles.
It estimated the apogee at 2,796 miles.
Japan's chief cabinet secretary Yoshihide Suga said the missile landed in the Sea of Japan.
He called the provocation unacceptable and said Tokyo has filed a strong protest.
In Washington, White House press secretary Sarah Sanders tweeted that Mr Trump was briefed on the situation "while missile was still in the air".