North Korea fired a ballistic missile on Sunday, drawing a strong rebuke from US President Donald Trump who vowed "100 percent" support for key ally Japan at a press conference with Prime Minister Shinzo Abe.

The missile, the first test since Trump became president, was launched around 7:55 am (2255 GMT Saturday) from Banghyon air base in the western province of North Pyongan, and flew east towards the Sea of Japan (East Sea), the South Korean defence ministry said.

It flew about 500 kilometres (310 miles) before falling into the sea, a ministry spokesman said, adding the exact type of missile had yet to be identified.

"Today's missile launch... is aimed at drawing global attention to the North by boasting its nuclear and missile capabilities", the ministry said in a statement.

"It is also believed that it was an armed provocation to test the response from the new US administration under President Trump," it added.

Trump responded with an assurance to visiting Japanese Prime Minister Abe that Washington was committed to the security of its key Asian ally.

"I just want everybody to understand and fully know that the United States of America stands behind Japan, its great ally, 100 percent," Trump said, without elaborating further.

Abe denounced the launch as "absolutely intolerable" while top government spokesman Yoshihide Suge told reporters in Tokyo that it was "clearly a provocation to Japan and the region".

North Korea is barred under UN resolutions from any use of ballistic missile technology but six sets of UN sanctions since Pyongyang's first nuclear test in 2006 have failed to halt its drive for what it insists are defensive weapons.

'Clear provocation'
Last year the country conducted numerous tests and launches in its quest to develop a nuclear weapons system capable of hitting the US mainland.

A South Korean army official quoted by Yonhap news agency ruled out the possibility of a long-range missile test, describing the device as an upgraded version of the North's short-range Rodong missile.

Seoul-based academic Yang Moo-Jin said the latest test was "a celebratory launch" to mark the February 16 birthday of Kim Jong-Il, former ruler and father of North Korea's leader Kim Jong-Un.

Pyongyang often celebrates key anniversaries involving current and former leaders with missile launches, Yang, a professor at the University of North Korean Studies, told AFP.

South Korea's acting president Hwang Gyo-Ahn vowed a "corresponding punishment" in response to the launch, which came on the heels of a visit to Seoul by new US Defense Secretary James Mattis last week.

Mattis had warned Pyongyang that any nuclear attack would be met with an "effective and overwhelming" response.

In January leader Kim Jong-Un boasted that Pyongyang was in the "final stages" of developing an intercontinental ballistic missile (ICBM) in an apparent attempt to pressure the incoming US president. Trump shot back on Twitter, saying "It won't happen."

James Char, senior analyst at the Institute of Defence and Strategic Studies in Singapore, said the launch was Pyongyang's "way of showing characteristic defiance against... Trump".

Test for Trump
Washington has repeatedly vowed that it will never accept North Korea as a nuclear-armed nation and the latest launch poses a test for Trump, who will need the help of Beijing, Pyongyang's closest ally, to deal with the reclusive state.

Relations between the two superpowers have thawed in recent days after Trump reaffirmed Washington's "One China" policy in what he described as a "very warm" telephone conversation with President Xi Jinping.

The US leader pledged to honour a decades-old position that effectively acknowledges Taiwan is not separate from China -- a policy that Trump had suggested a few weeks ago he might jettison, angering Beijing.

"The recent Trump-Xi phone call would be considered an important platform from which the two powers will move forward," Char said.

Analysts are divided over how close Pyongyang is to realising its full nuclear ambitions, especially as it has never successfully test-fired an ICBM.

But all agree it has made enormous strides in that direction since Kim took over after the death of his father in December 2011.