Mattis’s threat came as Vice President Mike Pence toured an intercontinental ballistic missile launch site at an Air Force base in Minot, North Dakota, telling airmen that the Trump administration was prepared to launch an attack on North Korea.

Vice President Mike Pence: “The threats we face today mean that, once again, America’s security and our very future depend on the airmen of Minot being ready and being prepared. … History attests the surest path to peace is through American strength. There’s no greater element of American strength, there’s no greater force for peace in the world, than the United States nuclear arsenal.”

Pence’s visit came as the Trump administration seeks to expand the U.S. nuclear arsenal. A draft of the Nuclear Posture Review reviewed by top administration officials last month calls for new “low yield” nuclear warheads, sea-launched nuclear-armed cruise missiles, preparations for possible nuclear weapons tests and an easing of conditions under which the U.S. might launch a nuclear attack.