President Donald Trump told Philippine President Rodrigo Duterte in a phone call last month that two nuclear submarines were somewhere in the waters near North Korea.

"We have a lot of firepower over there," Trump said of the Korean Peninsula, according to a transcript of the call obtained by the New York Times and verified by White House officials.

"We have two submarines — the best in the world," Trump said, according to The Times. "We have two nuclear submarines, not that we want to use them at all."

Business Insider previously reported that the nuclear-powered USS Michigan submarine would head to North Korea's coast, joining the USS Carl Vinson carrier strike group. The Michigan does not carry nuclear weapons, but it does carry Tomahawk missiles — the same type used in the April 7 strike on a Syrian air base. It also carries special forces, and South Korean media has reported that Navy SEALs are in the region and may be training to take out North Korea's leadership.

All US submarines are nuclear-powered — though at any time, four or five are nuclear-armed and on "hard alert, in their patrol areas, awaiting orders for launch," Stephen Schwartz, the author of "Atomic Audit: The Costs and Consequences of US Nuclear Weapons Since 1940," told Business Insider.

It is unclear whether Trump was referring to the USS Michigan or the USS Cheyenne, another nuclear-powered but not nuclear-armed submarine in the region, or to another submarine armed with nuclear weapons. The locations of nuclear-armed submarines have to remain a secret to even the highest commanders in the US military.

However, even if Trump were discussing nuclear-armed submarines, it is highly unlikely his disclosure was inappropriate, because he did not appear to give specifics.

SEALs and divers from SEAL Delivery Vehicle Team 1 swim back to the guided-missile submarine USS Michigan. US Navy photo by Mass Communication Specialist 3rd Class Kristopher Kirsop

Nuclear-armed submarines make up the most secretive part of the US's nuclear deterrent. Essentially, even if an attacker could somehow neutralize all the US's nuclear weapons on land, they'd be unable to find the submarines. As a matter of national security, only the captains and crew of the submarines know for sure where they're located.

"In fact, we don't even know where they are — they run silent," Schwartz said.