North Korea fired an intermediate-range ballistic missile over Japan on Tuesday, prompting Tokyo to activate the country's missile alert system and order people to take shelter.

The last time North Korea fired a missile over Japan was in 2017, at the height of a period of "fire and fury" when Pyongyang's leader Kim Jong Un traded insults with then-US president Donald Trump.

South Korea's military said it had detected the launch of an IRBM, which flew around 4,500 kilometers (2,800 miles) at an altitude of about 970 kilometers and a speed of around Mach 17.

"Specific details are under close analysis by South Korean and US intelligence," the South's Joint Chiefs of Staff added in a statement.

South Korea's President Yoon Suk-yeol called the launch a "provocation" that violated UN regulations.

Yoon "ordered a stern response and to take corresponding measures in cooperation with the United States and the international community", his office said in a statement.

North Korea was not responding to routine daily contact on the inter-Korean liaison line Tuesday, Seoul's unification ministry said.

Tokyo also confirmed the launch of an IRBM, activating the country's missile alert warning system and urging people to take shelter.

"North Korea appears to have launched a missile. Please evacuate into buildings or underground," the government said in an alert issued at 7:29 am (2229 GMT Monday).

Prime Minister Fumio Kishida told reporters that the launch was "an act of violence following recent repeated launches of ballistic missiles. We strongly condemn this."

Defense Minister Yasukazu Hamada said that North Korea "has in the past launched Hwasong 12-type missiles four times, so this could be the same type".

If so, the flight distance, which Tokyo estimated at 4,600 kilometers, was thought to be a new record for that particular type of missile.

The last two times North Korea fired Hwasong-12 missiles over Japan, in September and August 2017, they travelled 3,700 kilometers and 2,700 kilometers respectively, Chad O'Carroll of Seoul-based specialist site NK News wrote on Twitter.

"This is the 8th test of the Hwasong-12 and the 3rd time it has overflown Japan," Jeffrey Lewis of the Middlebury Institute of International Studies wrote on Twitter.

Washington condemned the launch, with US National Security Advisor Jake Sullivan speaking to his counterparts in Seoul and Tokyo to discuss an "appropriate and robust" joint response.

"Sullivan reinforced the United States' ironclad commitments to the defense of Japan and the ROK (South Korea)," national security spokeswoman Adrienne Watson said in a statement.

Military drills

With talks long-stalled, nuclear-armed North Korea has doubled down on Kim's military modernization plans this year, testing a string of banned weaponry, including an intercontinental ballistic missile also for the first time since 2017.

Last week, Pyongyang fired short-range ballistic missiles on four occasions, including just hours after US Vice President Kamala Harris flew out of Seoul.

Pyongyang's latest bout of intense weapons testing comes as Seoul, Tokyo, and Washington ramp up joint military drills to counter growing threats from the North.

South Korea, Japan, and the United States staged anti-submarine drills Friday — the first in five years — just days after Washington and Seoul's navies conducted large-scale exercises in waters off the peninsula, involving a nuclear-powered US aircraft carrier.

Such drills infuriate North Korea, which sees them as rehearsals for an invasion.

Harris toured the heavily fortified Demilitarized Zone that divides the peninsula while on a trip that aimed to underscore her country's "ironclad" commitment to South Korea's defense against the North.

Washington has stationed about 28,500 troops in South Korea to help protect it from the North.

Significant escalation

Firing a missile over Japan represented a "significant escalation" by North Korea, said Leif-Eric Easley, a professor at Ewha University in Seoul.

"Pyongyang is still in the middle of a provocation and testing cycle," he added.

"The Kim regime is developing weapons such as tactical nuclear warheads and submarine-launched ballistic missiles as part of a long-term strategy to outrun South Korea in an arms race and drive wedges among US allies."

South Korean and US officials have also been warning for months that Kim is preparing to conduct another nuclear test, saying last week that this could happen soon after China's upcoming party congress on 16 October.

North Korea, which is under multiple UN sanctions for its weapons programs, typically seeks to maximize the geopolitical impact of its tests with careful timing.

The isolated country has tested nuclear weapons six times since 2006, most recently in 2017.