SEOUL, South Korea (AP) — In an exchange of threats, U.S. President Donald Trump warned Pyongyang of “fire and fury like the world has never seen” and the North’s military claimed Wednesday it was examining plans for attacking Guam.

The high-level tit-for-tat follows reports that North Korea has mastered a crucial technology needed to strike the United States with a nuclear missile.

Despite regular North Korean threats against Guam, a U.S. territory in the Pacific about 2,100 miles (3,400 kilometers) from the Korean Peninsula, it is extremely unlikely that Pyongyang would risk the assured annihilation of its revered leadership with a pre-emptive attack on U.S. citizens. It’s also not clear how reliable North Korea’s mid-range missiles would be in an attack against a distant target given the relatively few times they’ve been tested.

Even so, the competing threats and Trump’s use of North Korea-style rhetoric — Pyongyang has long vowed to reduce Seoul to a “sea of fire” — raise already high animosity and heighten worries that a miscalculation might spark conflict between the rivals.

The North Korean army said in a statement that it is studying a plan to create an “enveloping fire” in areas around Guam with medium- to long-range ballistic missiles. The statement described Andersen Air Force Base on Guam as a “beachhead” for a potential U.S. invasion of North Korea it needed to neutralize. It was unlikely the North’s threat was a direct response to Trump’s comments to the camera at his golf course in Bedminster, New Jersey.

Trump spoke hours after reports indicated North Korea can now wed nuclear warheads with its missiles, including its longest-range missiles that may be able to hit the American mainland. The North has strived for decades to have the ability to strike the U.S. and its Asian allies, and the pace of its breakthroughs is having far-reaching consequences for stability in the Pacific and beyond.

The nuclear advances were detailed in an official Japanese assessment Tuesday and a later Washington Post story that cited U.S. intelligence officials and a confidential Defense Intelligence Agency report. The U.S. now assesses the North Korean arsenal at up to 60 nuclear weapons, more than double most assessments by independent experts, according to the Post’s reporting.

“North Korea had best not make any more threats to the United States,” said a stern-looking Trump, seated with his arms crossed and with his wife beside him. “They will be met with fire and fury like the world has never seen.”

“He has been very threatening beyond a normal state. And as I said they will be met with fire, fury and frankly power the likes of which this world has never seen before.”