United Nations Secretary-General Antonio Guterres is troubled by the "increase in confrontational rhetoric" on North Korea, a spokesman says.
"The Secretary-General remains extremely concerned by the ongoing situation and is troubled by the increase in confrontational rhetoric," UN spokesman Stephane Dujarric told reporters on Wednesday.
It comes following an exchange of threats and boasts of nuclear power by President Donald Trump and North Korea.
North Korea said it was considering plans for a missile strike on the US Pacific territory of Guam, after Mr Trump on Tuesday said any North Korean threat to the United States would be met with "fire and fury".
On Wednesday Mr Trump tweeted the US nuclear arsenal was "stronger and more powerful than ever before" but he hoped it never needed to be used.
US Secretary of State Rex Tillerson sought to play down the rhetoric. Shortly before Mr Trump's remarks on the nuclear arsenal, Mr Tillerson landed in Guam for a previously scheduled visit after telling reporters he did not believe there was an imminent threat from North Korea and that "Americans should sleep well at night".
With his "fire and fury" warning, Mr Trump was trying to speak in a way that would resonate with North Korean leader Kim Jong-un, Tillerson said.
Earlier on Wednesday, North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a US military base that includes a submarine squadron, an airbase and a Coast Guard group.
The plan would be put into practice at any moment, once Kim Jong-un made a decision, a Korean People's Army spokesman said in a statement carried by state-run KCNA news agency.
Guam Governor Eddie Calvo dismissed the threat and said the island was prepared for "any eventuality".
The United States and South Korea remain technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.
Tension in the region has risen since North Korea carried out two nuclear bomb tests last year and two intercontinental ballistic missile tests in July. Trump has said he will not allow Pyongyang to develop a nuclear weapon capable of hitting the United States.
North Korea, which is pursuing missile and nuclear weapons programs in defiance of UN Security Council resolutions, accuses the United States of devising a "preventive war" and has said that any plans to execute this would be met with an "all-out war, wiping out all the strongholds of enemies, including the US mainland".
China, which is North Korea's closest ally despite its anger at Pyongyang's missile and nuclear programs, described the situation as "complex and sensitive," and urged calm and a return to talks.
The South Korean capital, Seoul, is within range of massed North Korean rockets and artillery, which would be impossible to destroy in a first US strike.
Tens of thousands of US troops remain stationed in South Korea and in nearby Japan, the only country to have been attacked with nuclear weapons.