Mike Pence has called North Korea's failed missile launch a "provocation" after landing in South Korea for a 10-day tour of Asia.
The US Vice President arrived in the region a day after North Korea paraded missiles and military hardware, warning America of advancements in its nuclear and defence capabilities.
"This morning's provocation from the North is just the latest reminder of the risks each one of you face 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," he told troops at a military base in Seoul.
Mr Pence said America's resolve to help South Korea "has never been stronger", adding: "Under President Trump's leadership, we're going to rebuild our military."
North Korea attempted to fire a missile which exploded during its launch, US and South Korean officials said – a high-profile failure that comes as a powerful US aircraft supercarrier approaches the Korean Peninsula in a show of force.
Donald Trump described the carrier group as an "armada" as he indicated the US will toughen its stance towards North Korea, tweeting last week: "If China decides to help, that would be great. If not, we will solve the problem without them!"
The failed missile launch followed a huge military display in Pyongyang, the hermit kingdom's capital, to mark the birth anniversary of the nation's founding president Kim Il-sung.
What appeared to be new inter-continental ballistic missiles were unveiled at the parade, as a close aide to dictator Kim Jong-un warned that Mr Trump's actions could unleash nuclear war.
“If the United States wages reckless provocation against us, our revolutionary power will instantly counter with annihilating strike, and we will respond to full-out war with full-out war and to nuclear war with our style of nuclear strike warfare,” Choe Ryong-hae told crowds.
North Korea has conducted five nuclear tests, including two last year. Recent satellite imagery suggests the country could conduct another underground nuclear test at any time.
The White House said it believed the latest test involved a medium-range ballistic missile that failed within four to five seconds after launch, and that it did not involve an inter-continental ballistic missile.
The missile blew up "almost immediately" after it was attempted at 11.21 Hawaii time – 9.21pm UK time – from the east coast city of Sinpo, said US Navy Commander Dave Benham.
The North regularly launches short-range missiles, but is also developing mid-range and long-range missiles meant to target US troops in Asia and, eventually, the US mainland.
After his arrival in Seoul, Mr Pence placed a wreath at Seoul National Cemetery and then worshipped with military personnel at an Easter church service at the US Army Garrison Yongsan.
During a meal after the services, he said the tensions on the Korean peninsula had put into sharp focus the importance of the joint US-South Korean mission.
“Our commitment to this historic alliance with the courageous people of South Korea has never been stronger," said the Vice President.
“With your help, and with God's help, freedom will ever prevail on this peninsula.”
Mr Pence told the military members that he had spoken twice with Mr Trump during the day.
China’s foreign minister urged both the US and North Korea to de-escalate the situation before it gets to an “irreversible and unmanageable stage” on Friday, calling on both sides to “refrain from provoking and threatening each other”.
“If a war occurs, the result is a situation in which everybody loses and there can be no winner,” Wang Yi warned.
Shannon Kile, a nuclear specialist at the Stockholm International Peace Research Institute, said North Korea had previously deployed “mock-ups” of missiles at its parades and it was difficult to verify the weapons on show.
He told The Independent what appeared to be the KN-08 ICBM was rolled out, although it has not yet been flight tested, as well as large canisters indicating the possible development of a “cold launch” long-range missile.
Additional reporting from agencies
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