North Korea has said it is examining its operational plans for attacking Guam, to contain US bases there.
In a statement distributed by the state-run news agency, the army said the move was in response to a recent US intercontinental ballistic missile test and it was studying a plan to create an "enveloping fire" in areas around Guam with medium to long-range missiles.
The US territory is home to Andersen Air Force Base.
It comes as US president Donald Trump said North Korea "will be met with fire and fury like the world has never seen" if it made any more threats against America.
Mr Trump issued the warning during a briefing on opioid addiction at his golf course in Bedminster, New Jersey.
A Japanese defence paper and a US media report said on Tuesday that North Korea may have successfully produced a miniaturised nuclear warhead that can fit inside its missiles - a key benchmark in the country’s attempt to become a full-fledged nuclear power.
Washington’s alarm over North Korean leader Kim Jong Un’s pursuit of a nuclear capability has intensified in the past month after the North conducted two tests of intercontinental ballistic missiles last month.
But Republican senator John McCain said "great leaders" did not threaten foes unless they were ready to act and he was not certain Mr Trump was.
Mr McCain, who heads the Armed Services Committee, made the comments in an interview on KTAR News in Phoenix.
He said he took exception to Mr Trump’s comments "because you got to be sure you can do what you say you’re going to do".