United States Vice President Mike Pence put North Korea on notice on Monday, saying neither the US nor South Korea would tolerate further missile and nuclear tests.
He was visiting the heavily mined, four-kilometre-wide (2.5-mile-wide) demilitarised border between North and South Korea, a day after North Korea's failed missile launch.
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 region.
Tensions between Pyongyang and Washington have soared in recent weeks, as a series of North Korean missile tests have prompted ever more bellicose warnings from Trump's administration.
"Just in the past two weeks, the world witnessed the strength and resolve of our new president in actions taken in Syria and Afghanistan," Pence said.
"North Korea would do well not to test his resolve or the strength of the armed forces of the United States in this region," Pence said at a press conference with South Korea's acting President Hwang Kyo-Ahn.
Standing by South Korea risks alienating China
Pence, whose father served in the 1950-53 Korean War, said the US would stand by its "iron-clad alliance" with South Korea and sought peace through strength.
The US, its allies and China are working together on a range of responses to North Korea's latest failed ballistic missile test, Trump's national security adviser said on Sunday, citing what he called an international consensus to act.
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.
But Pence and Hwang said they were troubled by retaliatory economic moves by China against the deployment in South Korea of a US anti-missile system known as the Terminal High Altitude Area Defense (THAAD).
South Korea, which accuses China or discriminating against some South Korean companies working in China, and the United States say the sole purpose of THAAD is to guard against North Korean missiles.
China says its powerful radar can penetrate its territory and undermine its security and spoke out against it again on Monday.
Chinese Foreign Ministry spokesman Lu Kang told reporters in Beijing the situation on the Korean peninsula was "highly sensitive, complicated and high risk", adding all sides should "avoid taking provocative actions that pour oil on the fire".
Pence's visit came a day after North Korea held a military parade in its capital, Pyongyang, marking the 105th anniversary of the birth of founding father Kim Il-sung.