U.S. Secretary of Defense Ashton Carter was to arrive in Seoul Thursday as the U.S. and South Korea demonstrated their combined military heft, seeking to deter North Korea from making good on threats to test another nuclear device.

Carter's three-day visit comes as the allies' most extensive live-fire drills of the year still ring in the ears of the regime in Pyongyang. North Korea has slammed the Foal Eagle exercises near the border as agitation for nuclear war. The U.S. and South Korea say they are purely defensive.

"In Korea, I'll be working with my counterpart to reinforce deterrence and improve capabilities on the peninsula to counteract an increasingly dangerous and provocative North Korea," Carter said on Monday at a speech at Arizona State University before his trip to Japan and South Korea.