Donald Trump has said "all options are on the table" after North Korea launched a ballistic missile over Japan.
The US president said in a written statement that "threatening and destabilising actions" only increase North Korea's isolation in the region and around the world.
He added that North Korea's actions show "contempt for its neighbours" and that "all option are on the table" for a US response.
North Korea fired a mid-range ballistic missile, designed to carry a nuclear payload, from its capital Pyongyang that flew over Japan on Tuesday before plunging into the northern Pacific Ocean, in what appeared to be its longest-ever test.
The test-flight over the territory of a close US ally sent a clear message of defiance as Washington and Seoul conduct war games nearby.
South Korea's joint chiefs of staff said the missile travelled around 1,677 miles and reached a maximum height of 341 miles as it flew over the northern Japanese island of Hokkaido.
The launch, which seems to be the first to cross over Japan since 2009, will concern a region worried that each new missile test puts the North a step closer towards its goal of an arsenal of nuclear missiles that can reliably target the US.
North Korean launches have been happening at an unusually fast pace this year, and some analysts believe Pyongyang could have viable long-range nuclear missiles before the end of Mr Trump's first term in early 2021.
The South Korean military said it is analysing the launch with the US and has strengthened its monitoring and preparation in case of further actions from North Korea.
Analysts speculate the North may have tested a new intermediate-range missile that Pyongyang recently threatened to fire towards the US territory of Guam, which hosts a major military base.
This missile landed nowhere near Guam, which is about 1,550 miles south of Tokyo, but the length of Tuesday's launch may have been designed for the North to show it could follow through on its threat.
Seoul says the missile was launched from Sunan, which is where Pyongyang's international airport is, opening the possibility that North Korea launched a road-mobile missile from an airport runway.
Japanese officials said there was no damage to ships or anything else reported. Japan's NHK TV said the missile separated into three parts.
"We will do our utmost to protect people's lives," prime minister Shinzo Abe told reporters.
"This reckless act of launching a missile that flies over our country is an unprecedented, serious and important threat."
The launch came days after the North fired what were assessed as three short-range ballistic missiles into the sea and a month after its second flight test of an intercontinental ballistic missile, which analysts said could reach deep into the US mainland when perfected.
It was North Korea's 13th launch of ballistic missiles this year, said Roh Jae-cheon, spokesman for Seoul's joint chiefs of staff.
South Korea's foreign ministry warned that the North will face a "strong response" from the US-South Korean alliance if what it called nuclear and missile provocations continue.