The comments come after a failed missile test launch by North Korea, and a massive military parade.
The US Navy dispatched the aircraft carrier USS Carl Vinson strike group to the region last weekend, and US President Donald Trump has been tweeting this week that if China can't rein in North Korea's nuclear program, the United States will.
Just as Vice President Mike Pence took off for South Korea late Saturday, Kim Jong Un's regime in neighboring North Korea test-fired a new missile.
That was the site of a ballistic missile test earlier this month in which the projectile fell into the Sea of Japan, also known as the East Sea.
The failed test occurred as a USA naval strike force moves into the region in a show of force, and amid increasingly tough warnings from the Trump administration that the U.S.is prepared to take increased measures against North Korea that include possible military action. Given the day's importance in North Korean society, many experts anticipated that the celebration would bring with it another nuclear test.
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.
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"Our commitment to this historic alliance with the courageous people of South Korea has never been stronger".
He has repeatedly said if China, North Korea's dominant trading partner, is unwilling to do more to pressure the North, the USA might take the matter into its own hands.
North Korea shows off its missiles at a military parade in Pyongyang on April 15, 2017, in a photo taken from North Korean TV footage.
Pence said Sunday at a fellowship dinner at U.S. Army Garrison Yongsan in Seoul that the willingness of military members "to stand firm without fear inspires the nation and inspires the world".
Some of the weapons on show included nuclear-capable missiles that have previously been successfully tested.
There were no further details, and the office said it was analysing the launch.
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A national security official traveling with Pence told the reporters accompanying him that that the USA monitored the missile launch closely and that it failed after about five seconds.
Kim Jong-un oversaw the festivities on the "Day of the Sun" at Pyongyang's main Kim Il Sung Square.
"Why would I call China a currency manipulator when they are working with us on the North Korean problem?"
Seoul's foreign ministry "strongly condemned" the launch, warning that Pyongyang would face "serious punitive measures" should it conduct a nuclear test or other major strategic provocations.
Despite Sunday's failure, the North's previous claim to have used "standardized" warheads has led to worries that it was making headway in its push to develop small and sophisticated warheads to be topped on long-range missiles.
North Korea has conducted five nuclear tests, including two a year ago.
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