US Secretary of State John Kerry delivered a stark warning to North Korea not to test-fire a mid-range missile.
He also damped down anxiety caused by a new intelligence report suggesting significant progress in the communist regime's nuclear weapons programme.
Kicking off four days of talks in an east Asia beset by increasing North Korean threats, Mr Kerry told reporters in Seoul that Pyongyang and its enigmatic young leader would only increase their isolation if they launched the missile that American officials believe has a range of about 2,500 miles - enough to reach the US territory of Guam.
"If Kim Jong Un decides to launch a missile, whether it's across the Sea of Japan or some other direction, he will be choosing willfully to ignore the entire international community," Mr Kerry said.
"And it will be a provocation and unwanted act that will raise people's temperatures."
If the trajectory of the test missile suggests that it could be a threat to either the US or allies, the military would move to shoot it down from one of nine warships armed with sophisticated ballistic missile defence systems in the Pacific, including two that were moved closer to the Korean peninsula, US officials said.
Mr Kerry said the test would be a "huge mistake" for Kim.
"It will further isolate his country and further isolate his people who are desperate for food and not missile launches," he warned. "They are desperate for opportunity and not for a leader to flex his muscles."
Mr Kerry's diplomatic tour, while planned long in advance, is unusual in that it brings him directly to a region of escalated tensions and precisely at a time when North Korea is threatening action.
The North often times its military and nuclear tests to generate maximum attention, and Mr Kerry's presence on the peninsula alone risked spurring Pyongyang into another provocation. Another key date is the 101st birthday of the nation's founder, Kim Il Sung, on Monday.
After meeting South Korean president Park Geun-hye and foreign minister Yun Byung-se, Mr Kerry also addressed an intelligence report that rocked Washington on Thursday, suggesting that North Korea now had the know-how to arm a ballistic missile with a nuclear warhead - even if the weapons would lack reliability.
Mr Kerry, repeating assertions by other administration officials, noted that Pyongyang still had not developed or fully tested the nuclear capacities needed for such a step.
He offered strong words of solidarity for South Korea, praising Mr Park's "bright vision" of a prosperous and reunified Korean Peninsula without nuclear weapons.
By contrast, he said North Korea's Kim, by estimates only 29 or 30 years old, had a choice to make between provocation and returning to talks to de-escalate tension and lead to the end of its nuclear programme.
"It's up to Kim Jong Un what he decides to do," Mr Kerry said.
A missile launch, he said, "is not going to change our current position which is very clear: We will defend our allies. We will stand with South Korea and Japan against these threats. And we will defend ourselves."
Mr Yun called for more United Nations action against Pyongyang if it committed another provocation.
He refused to comment specifically on the US intelligence report, saying only that the North had "high nuclear and missile capabilities" but that it was still some time away from a nuclear bomb that is "small, light and diversified".
Both Mr Yun and Mr Kerry kept the door open for future negotiations with Pyongyang, but seemed to suggest that such talks were unlikely in light of the North's increasingly bombastic threats, including nuclear strikes on the United States.
Most experts say those are unfeasible based on the North's current capacity and would never be explored seriously because the US response would be overwhelming against a regime focused primarily on survival.
Mr Kerry said any talks with North Korea have to lead toward de-nuclearisation.
They have to be really serious," he said. "No one is going to talk for the sake of talking and no one is going to play this round-robin game that gets repeated every few years, which is both unnecessary and dangerous."
Meanwhile North Korea has denied involvement in a cyber-attack that shut down nearly 50,000 computers and servers at South Korean broadcasters and banks last month.
The General Staff of the Korean People's Army issued the denial today through the official Korean Central News Agency.
Seoul said on Thursday that Pyongyang was responsible for the March 20 computer crash.