Visiting American Vice President Mike Pence described North Korea's failed missile test on Sunday as a "provocation" and assured South Korea of Washington's full support against the threat from its volatile neighbour.
Pyongyang launched the missile hours before Pence arrived in Seoul for talks on curbing the North's weapons programmes as fears grow that it is planning another nuclear weapon test.
But US officials said the missile exploded seconds after it was fired.
"This morning's provocation from the North is just the latest reminder of the risks each one of you face each and every day in the defence of the freedom of the people of South Korea and the defence of America in this part of the world," Pence said.
Some 28,500 US troops are stationed in the South.
North Korea launches missile despite US tensions
The latest launch came a day after the North held a defiant massive military parade in Pyongyang that showcased nearly 60 missiles - including a suspected new intercontinental ballistic missile.
"The missile blew up almost immediately," the US military said of Sunday's early morning launch from near Sinpo on the North's east coast.
The type of missile was not clear.
The North has a habit of test-firing missiles to mark major dates such as Saturday's 105th anniversary of the birth of the nation's founder Kim Il-sung, or as gestures of defiance when top US officials visit the region.
South Korea's foreign ministry said by conducting the latest test just a day after displaying a series of missiles, "North Korea has threatened the whole world".
Last August a submarine-launched ballistic missile tested from Sinpo flew 500km towards Japan.
North Korean leader Kim Jong-un hailed that test as the "greatest success" and said it brought the US mainland within range of a mobile delivery system.
Pyongyang's rogue atomic ambitions have come into sharp focus in recent weeks, with Trump vowing a tough stance against the North and threatening unilateral action if China failed to help curb its neighbour's nuclear programme.
Trump has repeatedly said he will prevent Pyongyang from developing a nuclear-tipped ballistic missile capable of reaching the mainland United States.
With speculation mounting that the North is preparing to conduct a sixth nuclear test, he sent an aircraft carrier-led strike group to the Korean peninsula.
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The North has reiterated its constant refrain that it is ready for war with the United States, and its army on Friday vowed a "merciless" response to any US provocation.
Recent satellite images suggest its main nuclear site is "primed and ready", according to specialist US website 38North.
China, the North's sole major ally, and Russia have both urged restraint.
On Monday Pence is scheduled to meet acting-President Hwang Kyo-ahn for talks expected to focus on the North's weapons programmes and a controversial US missile defence system known as THAAD.
Source: AFP news agency