North Korea's deputy representative to the United Nations, Kim In Ryong, accused Washington of creating "a situation where nuclear war could break out an any time" and said Pyongyang's next nuclear test would take place "at a time and at a place where our headquarters deems necessary".
"The era of strategic patience is over".
Throughout his bareknuckle election campaign, US President Donald Trump repeatedly called into question a mutual defense treaty between Japan and the United States, suggesting Tokyo should pay for its own security.
"North Korea attempted to test an unidentified type of missile from [its eastern port of] Sinpo", the South Korean defence ministry said, adding that the launch on Sunday had "failed".
"The most important message from the vice president on behalf of the president is that we have an ironclad commitment to stand with our allies in the region, in their defense", Pence Press Secretary Marc Lotter said in an interview on ABC's "Powerhouse Politics" podcast ahead of the trip.
Meanwhile, China made a plea for a return to negotiations.
Tensions have escalated over North Korean moves to accelerate its weapons development. Kim said North Korea is once again justified in amplifying its nuclear program as the USA looks to bring down its socialist system.
He reiterated United States support for South Korea, telling his host: "We are with you 100%". "If they are unable to do so, the USA, with its allies, will!" he tweeted Thursday.
Speaking alongside the South's acting president, Hwang Kyo-ahn, Mr Pence said: "Just in the past two weeks, the world witnessed the strength and resolve of our new president in actions taken in Syria and Afghanistan".
The vice president earlier visited a military installation near the DMZ, Camp Bonifas, for a briefing with military leaders.
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It's four kilometers (2.5 miles) wide, stretches 250 kilometers (160 miles) and is dotted with military guard posts, mines and defensive structures. "But dialogue for the sake of having dialogue is meaningless".
"We support the U.S. putting pressure on Chinese banks to ensure that the elites in North Korea can not continue to live the comfortable lives that they now do while the people of North Korea are suffering".
Pence's visit came amid increasing tensions and heated rhetoric on the Korean Peninsula.
Pence assured the US would react with "an overwhelming and effective response" should North Korea attack.
The two later discussed how to rein in North Korea's nuclear and missile programmes.
Also on Sunday, Lt Gen HR McMaster, the United States top security adviser, said his country was working on a "range of options" with China, the first confirmation the two countries were co-operating to find a solution to the North Korean issue.
McMaster cited Trump's recent decision to order missile strikes in Syria after a chemical attack blamed on the Assad government, as a sign that the president "is clearly comfortable making tough decisions".
The North Koreans use Sinpo shipyard for their submarine activity, and USA satellites have observed increased activity there in April, a U.S. official said at the time of the previous test.
While China is North Korea's biggest economic lifeline, relations have been frosty and Beijing has a few times expressed frustration with Pyongyang.
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