The United States is ruling nothing out in its dealings with North Korea, Vice President Mike Pence said on Monday during a visit to the heavily militarised border dividing the two Koreas.
"Washington wants to achieve security "through peaceable means, through negotiations. But all options are on the table as we continue to stand shoulder to shoulder with the people of South Korea," he said at the border village of Panmunjom.
Pence said America's relationship with South Korea was "ironclad and immutable."
The people of North Korea the military of North Korea should not mistake the resolve of the United States of America to stand with our allies.
Pence's visit to the border comes a day after North Korea's latest missile test failed, when the rocket exploded seconds after blast off.
Tensions have risen as US President Donald Trump takes a hard rhetorical line with North Korean leader Kim Jong un, who has rebuffed admonitions from China and proceeded with nuclear and missile programmes seen by Washington as a direct threat.
Trump acknowledged on Sunday that the softer line he had taken on China's management of its currency was linked to Beijing's help on the North Korea issue.
"Why would I call China a currency manipulator when they are working with us on the North Korean problem? We will see what happens!" Trump said on Twitter. Trump has backed away from a campaign promise to label China in that way.
South Korea said the North's latest show of force "threatened the whole world."
Japanese Prime Minister Shinzo Abe urged North Korea to refrain from taking further provocative actions, comply with UN resolutions and abandon its nuclear missile development.
"Japan will closely cooperate with the US and South Korea over North Korea and will call for China to take a bigger role," Abe told parliament.
However, a US foreign policy adviser travelling with 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.
"We had good intelligence before the launch and good intelligence after the launch," the adviser told reporters on condition of anonymity.
China has spoken out against the North's weapons tests and has supported UN sanctions. It has repeatedly called for talks while appearing increasingly frustrated with the North.
Beijing banned imports of North Korean coal on Feb. 26, cutting off Pyongyang's most important export. China's customs department issued an order on April 7 telling traders to return North Korean coal cargoes, trading sources said.
Trump's decision to order a cruise missile strike on a Syrian airfield this month, in response to what he said was Syria's use of chemical weapons, raised questions about his plans for reclusive North Korea.
Pyongyang has conducted several missile and nuclear tests in defiance of UN sanctions and regularly threatens to destroy South Korea and the United States.