US Vice President Mike Pence, pledging an "unshakeable" commitment to South Korea, landed at a US military base next to the demilitarised border with North Korea, a day after a failed missile launch by the North.
Mr Pence flew to South Korea 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 (DMZ) is a heavily mined, four-km-wide strip of land lined with barbed wire running across the Korean peninsula, with soldiers on both sides in a continual eyeball-to-eyeball standoff.
Mr Pence, whose father served in the 1950-53 Korean War, said he was humbled to be at the DMZ and hailed the alliance with South Korea.
"It is a testament to the unshakeable bond between our people," he said.
The US, its allies and China are working together on a range of responses to North Korea's latest failed ballistic missile test, US President Donald Trump's national security adviser said on Sunday, citing what he called an international consensus to act.
HR McMaster indicated that Mr Trump was not considering military action for now, even as a nuclear-powered aircraft carrier strike group was heading for the region.
"It's time for us to undertake all actions we can, short of a military option, to try to resolve this peacefully," he said on ABC's This Week programme. "We are working together with our allies and partners and with the Chinese leadership to develop a range of options."
"There is an international consensus now, including the Chinese leadership, that this is a situation that just cannot continue," McMaster said.
The Trump administration is focusing its North Korea strategy on tougher economic sanctions, possibly including an oil embargo, a global ban on its airline, intercepting cargo ships and punishing Chinese banks doing business with Pyongyang, Reuters reported last week, citing US officials.
The North Korean missile blew up almost immediately after its test launch on Sunday, the US Pacific Command said.
Hours later, Mr Pence landed for talks on the North's increasingly defiant arms program.