US Secretary of Defense James Mattis, in a saber-rattling gesture aimed at Pyongyang, reportedly landed in South Korea on Friday aboard a US E-4B “doomsday plane.”

The converted Boeing 747 is an airborne command post that allows Mattis to communicate with US forces in the event of nuclear war.

Mattis and his entourage went to the Demilitarized Zone that separates the two Koreas on the same day, where the top US military official delivered a speech accusing North Korea of building a nuclear arsenal to “threaten others with catastrophe.” He said the Trump administration remains committed to compelling the North to accept complete nuclear disarmament and added that the US is solidly behind its ally South Korea.

“We stand shoulder to shoulder with you and the Korean people in confronting the threats posed by the Kim Jong-un regime,” Mattis said, as South Korean Defense Minister Song Young-moo stood beside him.

The Trump administration has orchestrated a show of force against North Korea over the last week. It now has three carrier strike groups operating in the western Pacific that can be used against the North. It also plans to deploy 12 advanced F-35A stealth fighters to Japan next month.

On the psy-war side, both Mattis and Song are expected to don combat uniforms when they mingle with their troops in South Korea and deliver a joint statement, in another message to the North.