U.S. Vice President Mike Pence has said all options are on the table to contain North Korea's nuclear weapons program.
This is Pence's first visit to South Korea - part of an Asia swing that will also include stops in Japan, Indonesia and Australia - and although it was conceived months ago, could hardly come at a time of higher tension.
In a statement on Monday alongside South Korea's acting president, Mr Pence said: "But the era of strategic patience is over".
Pence later stood a few yards from the military demarcation line outside Freedom House, gazing at two North Korean soldiers across the border and then a deforested stretch of North Korea from a lookout post in the hillside.
The U.S. Navy this month struck a Syrian airfield with 59 Tomahawk missiles.
McMaster said various USA military and intelligence agencies are working on providing options to "have them ready" for Trump "if this pattern of destabilizing behavior continues". "That the nuclear test did not happen will surely be seen as the pressure working".
Now, some estimates are that North Korea may have the capability to launch a missile that could hit the continental United States by the year 2020.
The words resonated in Japan and South Korea, both of which share the US' hardline stance on the North Korean issue.
Pence's visit has been full of Cold War symbolism and his comments came amid spiraling tensions on the Korean Peninsula.
"We're going to abandon the failed policy of strategic patience".
The foreign policy official said Sunday's test by North Korea was probably of a medium-range missile, and he signalled the U.S. was not likely to take action.
China has spoken out against the North's weapons tests and has supported United Nations sanctions. It has repeatedly called for talks while appearing increasingly frustrated with the North. But China furiously objects to its deployment, saying it could spy on its own defence installations, and has taken apparent retaliatory action against South Korean firms operating in its country.
U.S. Vice President Mike Pence landed on Sunday morning in Seoul in order to meet with South Korean officials and discuss how the two countries can deal with the North Korean issue.
China says its powerful radar can penetrate its territory and undermine its security and spoke out against it again on Monday.
Chinese Ministry of Foreign Affairs spokesman Lu Kang said Monday that Beijing's policy was "clear for all to see when it comes to promoting the denuclearization of the peninsula".
All sides should "avoid taking provocative actions that pour oil on the fire", he said.
Speaking Sunday, US national security adviser H. R. McMaster said the US hopes not to use military force but warned "this problem is coming to a head".
But at the same time he said: "It's time for us to undertake all actions we can, short of a military option, to try to resolve this peacefully".
After a two-month policy review, the Trump administration settled on a policy dubbed "maximum pressure and engagement", USA officials said Friday. Last week, Trump himself told The Wall Street Journal that he didn't actually understand China's position on North Korea until the president of China, Xi Jinping, explained it to him. Pence was briefed on the situation while flying to South Korea, reporter Jihye Leetells our newscast.
North Korea has spent decades trying to develop operational nuclear weapons. McMaster also noted that while the US hoped not to be forced to exercise its military capabilities in the region, he believed the "problem is coming to a head" and that the US is now working on options for a variety of courses of action for US President Donald Trump to choose from should "this pattern of destabilizing behavior" continue.