US Vice President Mike Pence has said that neither the United States nor South Korea would tolerate further missile and nuclear tests by North Korea.
Mr Pence, who is visiting South Korea, said defences against North Korea in the region would be strengthened.
He also said the US "era of strategic patience" with Pyongyang is over as he visits the demilitarised border between North and South Korea.
Mr Pence is on the first stop of a four-nation Asia tour intended to show America's allies, and remind its adversaries, that the Trump administration is not turning its back on the increasingly volatile region.
The demilitarised zone is a heavily mined, 4km-wide strip of land lined with barbed wire running across the Korean peninsula, with soldiers on both sides in a continual standoff.
Mr Pence, whose father served in the 1950-53 Korean War, said the United States would stand by its "iron-clad alliance" with South Korea and was seeking peace through strength.
"All options are on the table," Mr Pence said.
He said US President Donald Trump has made clear he will not talk about specific military tactics.
"There was a period of strategic patience but the era of strategic patience is over," he said.
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As Mr Pence and his wife, Karen, and daughters Charlotte and Audrey, visited an observation post, propaganda music floated across from the North Korean side.
The United States, its allies and China are working together on a range of responses to North Korea's latest failed ballistic missile test, President Trump's national security adviser said yesterday, citing what he called an international consensus to act.
Meanwhile, Mr Pence is to meet with Japan's deputy prime minister Taro Aso tomorrow in Tokyo, the second stop on his ten-day tour of Asia.
