President Donald Trump has said the US stands fully behind Japan in the aftermath of North Korea's latest missile launch.
"I just want everybody to understand, and fully know, that the United States of America is behind Japan, our great ally, 100%," Mr Trump said at a news conference with Japan's Prime Minister Shinzo Abe.
Mr Abe denounced the launch as "absolutely intolerable".
:: Key steps in North Korea's nuclear ambitions
The missile - launched at around 7.55am local time from Banghyon air base in the western province of North Pyongan Province - flew about 310 miles east towards the Sea of Japan before it dropped into water, the South Korean defence ministry said.
It is the first time the isolated state has tested such a device since Mr Trump took office.
"It is believed that today's missile launch... is aimed at drawing global attention to the North by boasting its nuclear and missile capabilities," the South's defence 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."
The Pentagon said the projectile was a medium- or intermediate-range ballistic missile.
The launch never posed a threat to North America, it added, but did not state whether it believed the launch had been a success or failure.
The South Korean military said it suspected the North might have been testing a intermediate-range Musudan missile.
Last October, North Korea test-fired Musudan missiles twice from the same air base.
The North has conducted two nuclear tests and a number of missile-related tests at an unprecedented rate since early last year.
North Korean leader Kim Jong-Un said in his New Year speech that the country was close to test-launching an intercontinental ballistic missile (ICBM) and state media have said such a launch could come at any time.
But President Trump shot back on Twitter, saying "It won't happen".
The move prompted a vow of an "overwhelming" response from US Defence Secretary James Mattis when he travelled to South Korea earlier this month.
The latest missile test comes after Mr Trump assured Mr Abe, who has been on a visit to the US, that Washington was committed to the security of its key Asian ally.
"We will work together to promote our shared interests, of which we have many, in the region, including freedom from navigation and defending against the North Korean missile and nuclear threat, both of which I consider a very, very high priority," Mr Trump said on Friday.