U.S. Vice President Mike Pence has visited the Korean Demilitarized Zone Monday and later said the "era of strategic patience is over" with North Korea.
On Saturday North Korea displayed almost 60 missiles - including what is suspected to be a new intercontinental ballistic missile - at a parade to mark the 105th birthday of its founder Kim Il-Sung.
Noting Trump's recent military actions in Syria and Afghanistan, Pence said, "North Korea would do well not to test his resolve" or the USA armed forces in the region.
However, a U.S. foreign policy adviser traveling with Mr Pence sought to defuse some of the tension, saying Sunday's test of what was believed to be a medium-range missile had come as no surprise.
North Korea is looking for trouble.
Some analysts were puzzled by this.
A North Korea missile exploded during launch in the latest test on Sunday. "They can stop this if they want to due to their control over the North Korean economy".
A North Korean missile launched near Sinpo on the east coast "blew up nearly immediately", according to US Pacific Command. It had repeated failures with other missiles, including the medium-range Musudan and a submarine-launched ballistic missile, before successfully firing both.
Mr Hallam said any direct bombing action of North Korea must be prevented, saying that although Kim Jong-un would lose a war, it would still cost lives.
After stopping in Seoul, Pence is to travel to Tokyo; Jakarta, Indonesia; and Sydney. He added that the alliance between South Korea and the United States is "ironclad".
Washington and Seoul are deploying the US Terminal High Altitude Area Defense system in the South to counter the North's missile threats, despite angry opposition from China.
Seoul and Washington agreed in July previous year to install one THAAD battery in southeastern South Korea. China strongly objects to the THAAD battery - in particular its radar system, which Beijing worries will be used to peer into China.More news: Petersburg metro bombing suspect detained by Russian police
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Mike Pence (second right) visits Observation Post Ouellette with his daughters near the truce village of Panmunjom in the Demilitarized Zone.
Congressman Mac Thornberry, McCain's counterpart in the House of Representatives, said Kim's message to the United States was "we are strong and we can hurt you".
"It's a critical point for the defense of South Korea in recognizing it's not an offensive weapon".
North Korea's ultimate goal is to have a full array of nuclear-tipped missiles in response to what Pyongyang says is hostility by Washington and Seoul meant to topple its government.
The complicating factor here is China.
Lawmakers said in television news interviews broadcast Sunday that China holds the key to resolving tensions with North Korea.
China on Friday warned that "storm clouds" were gathering. -South Korean military exercises being staged now are the largest-ever "aggressive war drill".
China's Foreign Ministry spokesman Lu Kang said Monday that tensions need to be eased on the Korean peninsula to bring the escalating dispute there to a peaceful resolution.
The North faced severe sanctions from the UN Security Council for a series of missile launches a year ago, in addition to two nuclear tests.
On Friday, North Korea said the unsafe security situation was due to the "Trump administration's reckless military provocation".
The US Navy dispatched the aircraft carrier USS Carl Vinson strike group to the region last weekend, and Trump has been tweeting this week that if China can't rein in North Korea's nuclear program, the United States will.
But Trump's national security adviser said "it's time for us to undertake all actions we can, short of a military option, to try to resolve this peacefully". "But for this particular case, if they took the time and energy to launch a missile that failed, we don't need to expend any resources against that". "That's always our concern", Lt. Gen. Thomas W. Bergeson, the number two US military officer in South Korea told ABC News' Martha Raddatz on This Week Sunday.