North Korea Sunday fired a ballistic missile into the sea in an apparent test of new United States (US) President Donald Trump, who responded by pledging “100 per cent” support for Washington's key regional ally Japan.

Japanese Prime Minister Shinzo Abe, whose country would be in range of any hostile North Korean launch, called the test “absolutely intolerable” during an impromptu press conference with Trump in Florida.

The missile 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's defence ministry said.

It flew about 500 kilometres (310 miles) before falling into the sea, a ministry spokesman said.

“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.

The South's Joint Chiefs of Staff said later the missile was “highly likely” to have been a modified intermediate-range Musudan missile.

It was the first such test since last October.

Trump, speaking alongside Abe, said Washington was committed to his country's security.

“I just want everybody to understand and fully know that the United States of America stands behind Japan, its great ally, 100 per cent,” he said, without elaborating.

Japan's top government spokesman Yoshihide Suge told reporters in Tokyo the test 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 two nuclear tests and numerous missile launches in its quest to develop a nuclear weapons system capable of hitting the US mainland.

Seoul-based academic Yang Moo-Jin said the latest test was “a celebratory launch” to mark the February 16 birthday of Kim Jong-Il, late ruler and father of current 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 US Defence Secretary James Mattis this month.

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

Trump's national security adviser Michael Flynn spoke to his South Korean counterpart Kim Kwan-Jin by phone and agreed to “seek all possible options” to curb future provocations by the North, Seoul's presidential office said.

Australian Prime Minister Malcolm Turnbull also condemned the launch as a “further threat to regional... peace and stability” and vowed to work with Beijing, Seoul and Tokyo to heap pressure on Pyongyang.

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

The latest launch poses a test for Trump, who will need the help of the North's closest ally China to deal with the reclusive state.

Relations have thawed in recent days after Trump reaffirmed Washington's “One China” policy in what he described as a “very warm” telephone conversation with Chinese President Xi Jinping.

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.